I've got to get in my two bits here.
I'd love to see a Sins2. I've been here from the beginning and before and have said it all along. When rebellion came out I peed my pants and I will again.
Main thing: Campaign.
This would be awesome. I'm torn between the two ideas here: show the alien race chasing the Vasari or not. If you do show the race, then I think it should be pretty epic. If you don't show the race, I don't know what the campaign is going to be about, but I almost think it should be character driven.
Another main thing: Multicore, 64 bit, and memory upgrades.
This would make the game able to run on a much larger scale. Which is the whole idea of sins, huge scale.
Orbiting planets and moons would be sweet, and maybe more "solar system like" systems (i.e. only 5-15 planets in a system with lots of suns being in a single map) The phase lanes could change when the planets moved.
Titans? Multiple for each faction? No. Unless they got a nerf or you could research so that frigates could withstand some of a Titan's fury, it would be no fun. Maybe a heavy capitol ship. Balance needs to be achieved.
MORE RESEARCH TIERS.
Kinda like Galactic Civilizations II where the tree is so extensive you have to specialize and choose your path.
A fancy ship designer.
I would love to see a ship designer with base hulls, extras, weapons, defenses, modules, the whole lot (also kinda like Galactic Civilizations II)(Yes I still play that game, sometimes). Being that this is a real time strategy, that could be hard but maybe you could partion off the screen and control units while taking some time to build your custom ships.
Ability to create custom fleet formations.
With new ships, maybe some are long range and some have very heavy defenses and very little firepower. You could tell the long range ships to always line up further back, while the heavy defense ships would take the brunt of the damage. This could involve a change in the game engine where ships that are "behind" other ships cannot be fired upon until the closer ships are taken out.
I like HLT's idea of not having to micromanage fleets as much. This ability to have fleets to disengage and retreat after achieving 25% losses (defensive posture), say, they would automatically retreat. If a capitol ship got to a certain damage percentage, it would automatically retreat. In an offensive posture, they would have to achieve 75% losses to retreat. Or you could tell them to "fight to the death".
Critical systems on frigates, cruisers, and capitol ships.
Engines, weapons, defensive suites, bridge, fighter/bomber bay, power plant, etc. These systems could be targeted (by the user/computer telling ships to target these systems in order to cripple ships) and could damage them so as to cripple the ship, i.e. if engines/propulsion were hit, then the ship would loose considerable speed and maneuvering capabilities. If the bridge were hit, the ship is no longer directly controlled by the user (i.e. it will auto attack) unless a retreat is ordered. If the fighter/bomber bay is hit, then fighter and bomber construction is stopped. If the power plant is hit, then hull regen, sheild regen, and antimatter regen is slowed. Certain weapons and defenses could be damaged and disabled in much the same fashion. Perhaps these "critical" hits could only be achieved after shields were brought down.
More races? Sure, as long as it doesn't get in front of any of this other stuff.
HOLY COW. IF TITANS HAD THIS. The Ragnarov would be freaking sick. The opening cinematic. Those railguns moving was awesome. HW2 thing, but good thing none the less.
Much also like the opening cinematic, if a piece of a ship gets cut off by a stray laser, then it floats away. Ok, maybe not that much, but more than just sparks and smoke. Please, come on here.
I'm sure I'll think of more, but either way. Sins2. Please. I beg of you.