My thanks to IC and stardock for doing another exp!!! 
I can already hear the gears turning in modders' heads, hehe... "Are we going to make the Battlestars into Titans?" "Are we going to make the ISDs into Titans?" "Are we going to make the Scimitar into a Titan?" "Rebels are PERFECT for the Galactic Civil War!"
My idea (or shall we say, dream ) of a Titan is basically the idea of infinite (or near infinite) forward momentum, like a snowball rolling down an infinitely large slope.
Key to my concept of Titans would be two new citizens of the Sins universe: modules, and crewmen.
A module is a structural component of the titan that provides one specialized function, and also always provides two things: added mass, and increased hitpoints.
Modules are built at capital ship factories, and have to be towed by a construction frigate to dock with a titan, and then be welded into the hull (which has an animation and a short "build time" about equivalent to creating a light cruiser).
Modules can be detached at any time, resulting in a loss of the hitpoints and module function, and the detached module just drifts aimlessly in space unless scuttled for resources or picked up by another titan. Detaching costs a small amount of metal and credits, and takes about as long as building an Arcova. The rest of the Titan can continue to function, but without propulsion, meanwhile. Enemy titans can pick up modules detached by their foes, or blow them to smithereens as they please.
The standard module types would include: one module type for each type of weapon (anti-capship, anti-structure, anti-frigate, anti-fighter, anti-planet); crew modules for additional crew supply; shielding modules (which provide shields in addition to the standard hitpoints); hangar modules for fighters; command modules (which unlock ability points to spend on activated abilities). On top of that, you can have specialized race or faction-specific modules that come at extra cost, for instance TEC would get armor modules, and Vasari could build a module for performing a phase jump on the interior of a gravity well.
Each additional module attached to a Titan reduces its overall speed and maneuverability, as well as increasing the time required to charge a phase jump. For balance, titans are always immune to phase jump debuffs and inhibitors, and system-disabling abilities only affect modules, not the engines. This is to give titans a fighting chance to jump; with many modules, it will take a titan perhaps minutes to charge its phase jump, and having it reset by one ability is a bit unfair.
Another dynamic aspect is that, while ordinary weapons fire damages the hitpoints of the ship as a whole, individual modules can be targeted by all activated abilities. This allows you to use disabling abilities (such as the Ion Cannon) to knock out a shield emitter or weapons module for a time. Activated abilities that deal damage will "incapacitate" a module: its hull will breach if the weapon is powerful enough, requiring the ship to sacrifice some crew to repair the damaged module over time. While the module is incapacitated, its hitpoints are removed from the ship's total, and its functions are disabled, but it still contributes to the titan's mass, and thus continues to slow it down. Directly targeting and disabling modules will be a primary way to start to bring down a titan.
Crewmen are acquired when inside a friendly or allied planet's gravity well, and they steadily come in until all modules are fully crewed. A module that is not fully crewed operates with an efficiency decrease equal to the proportion of lost crew versus the crew capacity. The rate at which crew are replenished from the planet depends on the population of the planet, but even a dead asteroid will provide some degree of re-crewing. Other than time invested, crewmen come for free any time your titan is within a friendly gravity well.
The base ship is the primary module of the titan, which contains a very large amount of crew, moderate defenses and good armor. The base ship does not use crew to perform its own functions; they are there to crew the modules. It is just slightly more powerful offensively than a level 10 battleship, but it costs much more money. However, without any modules, it is no match for a starbase, a swarm of bombers, or a fleet of heavy cruisers. The base ship contains minor anti-strike craft defenses, strong anti-heavy defenses (good at engaging capital ships), several large anti-planet torpedo bays, and 360 degree autocannon coverage against frigates (notably, no anti-structure weapons).
When modules are incapacitated or damaged by activated weapons, crew are channeled from the base ship's reserves to the module, eventually restoring it to capability. During the repairs, some crew will die due to the hazards of the job.
The last aspect of crew is that every Titan is capable of sending its crew down to an enemy planet for a hostile takeover attempt. While this is almost always quicker than a planetary siege, some crew die during the takeover, and takeover may fail if the titan commits too few crew to the mission -- the titan would then have to retreat to a friendly planet to acquire more crew. Hostile takeover maintains most of the existing planet occupant's population level, but accrues a temporary drop in productivity (build speed).
Each module, once welded into a titan, consumes a constant stream of credits and metal, which must be supplied by trade ships. The trade ships are willing to jump no more than three jumps from a friendly outpost to supply the titan, expandable to five via research. Modules that go unsupplied have three states of degradation. When trade ships drop off supplies, the modules in the most degraded state get priority. One trade ship making a delivery is enough to fully supply, say, three or four modules (depending on trade ship upgrades) for several minutes of game time.
The first state is a grace period, where the module can not be repaired if it is damaged or destroyed, but otherwise operates normally. This state lasts for 60 seconds of game time.
The second state is a degrading period, where the crew of the module will start to retreat from the module into the base ship, until only half the module's crew remain. The restrictions of the first state remain. This state lasts a further 120 seconds of game time.
In the last state, the module will be disabled. This is just like incapacitation, except that there is no repair time. The module simply shuts down and becomes dead weight. As soon as resources are available, the module will come back online as crew pour in, with no repair cost (unless the module was subsequently incapacitated while disabled).
The way this system works, the other major way of shutting down a Titan, aside from blowing up its modules, is to cut off its supply route. Empires deploying Titans will have to guard the entire phase jump path from the nearest outpost (where trade ships depart directly from the planet surface) to the Titan, to make sure that incoming trade ships are not destroyed. If an entire wave of trade ships is lost, a majority of the Titan's modules could be dead weight in minutes.
When first built, a titan would be about twice as large as a capital ship, and travel at about the same speed. Starting abilities and firepower would put it at a monetary disadvantage versus building capital ships or cruisers with the same amount of resources. The base ship, whether attached to modules or not, performs relatively like a capital ship, with activated abilities, experience levels, and its own propulsion and firepower.
A key aspect of my design here is that there is no limit to the number of modules, short of practicality. After a very large number of modules, the titan's speed would be so onerously slow as to make it basically a starbase. Its upkeep costs would be so prohibitively large as to shut down even a vast empire's limit on supply. The only other limit I can think of is that the base ship should cost more than one capital ship slot and a large amount of fleet supply.
A few miscellaneous mechanics:
*All factions can research an ability that makes all titans permanently visible on the map, regardless of game settings or otherwise. A small area around the titan is also visible, but not the whole gravity well it's in.
*Superweapons can target titans with their projectiles, but the fired projectile must actually hit the titan: a base ship with no modules (or a small number of them) could easily avoid a Novalith cannon fired from several jumps away. The advent culture bomb weapon will disable several of the titan's modules and cause a crew revolt (resulting in crew deaths); the Novalith cannon will utterly obliterate a number of modules, or shoot an unprotected base ship straight out of the sky if it doesn't evade the shot.
*Other than the total hitpoints counter for the entire ship, the primary module (base ship) is not vulnerable to direct attack unless it is the last module remaining. So you can destroy a titan with modules attached if you bring its hitpoints to 0, but you can't disable e.g. the weapons or propulsion of the base ship unless the other modules are destroyed or incapacitated.
*Titans with 20 modules attached become so big that the titan can construct its own modules inside a module building factory, which is (in principle) the last module that would have to be built from a shipyard for the titan.
*Each module comes out of the factory with a full crew complement needed for that module. The crew on the base ship and in crew modules are, effectively, spare crew -- in case existing crew are spent doing planet takeovers, lost in fights, etc.
*Although the most effective way to kill crew is to directly attack modules with offensive abilities, some crew will slowly die over time using a new stat called "Armor Mitigation". Armor mitigation is conceptually the opposite of shield mitigation; think of it like this: if the ship takes a large amount of damage in a short time, this is likely to rupture hulls and damage modules, killing crew. But if the ship takes an even larger amount of damage over a very long time, there will be time to repair damage and bring shields back online, before the damage can really penetrate the ship's defenses.
So here is a summary:
1. Titans start out as nothing special, and if you have no plans to module up, you may as well just build capital ships with your money.
2. Titans grow larger and larger over time as you add more modules, but they don't get faster; they can only get slower. The weight of committing a titan to a battle bears down heavily on your shoulders as you wait for its phase jump charge, knowing that it will likely be unable to retreat due to its speed if the enemy gets the best of it.
3. Titans with many modules are extraordinarily difficult to destroy using conventional weaponry: their extreme durability can survive sustained anti-structure, anti-capship and bomber fire in large quantities. Damaging them requires surgical strikes on the individual modules with powerful ship-mounted abilities, or cutting off its supply route. You could also run the hosting empire out of funds by capturing important economic planets, depriving them of the income needed at a strategic level to maintain the titan. This gets easier and easier the more modules they add.
Side note: I'm going to recommend that the final battleship type for each race be a dedicated offensive ability powerhouse: all of its abilities should in some way provide a targeted direct damage weapon. These can be used to effectively incapacitate titan modules, though at grave risk to any capital ship brave enough to stand toe to toe with a moduled-out titan!
