Basic ideas I made, while reading honor harrington and coming up with similar things. You've likely done a bunch about the arch already, but I figured I'd try and suggest some stuff anyway. As always, not sure if these are possible or not, so feel free to laugh at my lack of knowledge.
Pods: The first and most important change I envision, is that Arch shouldn't use fighters/bombers while fighting. Part of it is due to my current honor Harrington addiction, while the rest is due to make the Arch far different. Instead, while then using fighters/bombers, instead a large chunk of their defenses/ships have abilities based around creating 'pods' which are similar to the platform abilities used by some other races. Basically the pods would come in about four to five different types, with each type having two variations.
The Variations would basically be 'System Defense' vs Ship'. The main difference between the two comes down to different things. Namely, the 'System Defense' pods should be bigger and stronger with an 'unlimited' range but only fire one shot per creation. As the name suggests, these are generally made as an ability of the various types of tactical structures/possibly the specialized capitals. The idea behind them is that the Arch would fire their 'best' weapons first, rather then waiting until the confusion of true battle begins and they start to get lost in the confusion. Which in other words means the first salvo of the battle (from the arch side) would be their most deadly.
The normal pods on the other hand, would the type carried abroad the various ships. While not as deadly/long range as the system defense pods, they make up for it for having more shots and a lower recharge time, making them a more consistent weapon. To make up for this, they could possibly have the 'bomber' subtype, meaning that the enemies fighters and such would try and take care of them first. The various types of pods are what you would expect. Feel free to change the names or whatever.
Tea: Anti Bomber/Fighter Pods, the type of thing to counter an enemy bomber/carrier rush. Possibly mounts
Mocha: Anti Frigate Pods, mounts little missiles.
Cappa: Anti Heavy Cruiser/Captiial Pods. Most likely the ones most likely to be spammed and the like, but what can you do.
Double Expreos: Anti Building pods, range of something like the Ogliv cruiser. Generally not used by tactical structures (since it would counter the mobile starbases too much.) Possibly used by the offensive star base though.
Irish Coffa: Everyone's favorite holocaust causing pods. Attached to the Siege carrier and the Offensive Star base, kills planets. Simple but effective.
Iced Coffa: Boring pods, can technically hit anything but does very little damage to any. Used by the defensive star base and the Scout/Settlement captials.
Frappa: Repair pods. Not even sure if its possible, though would be interesting if so. Only source of repair ablities (still passive repair) in the Arch side, dropped in combat by the defensive starbase/coordinator podnought. Boosts shield regeneration/repair by 5% per pod.
Networking: A tech similar to the plague, that gets unlocked at level 3 and levels up up at five, seven and eight (but only after the seed AI at eight). Unlocks each buildings/ships networking bonus, which is shared with every allied thing in the gravity well. Shipyards speed up construction, Refineries increase income, Trade increases trade, etc etc.
Refinery/Trade in the same building. Not sure why, but it seems rather cool to me.
Culture Bonus's: ROF/Build speed increases:
Reasons: Since you seem to be basing them off the advent, that means special ability heavy while fragile. Since everyone loves a glass cannon, I figured them shooting and building faster when under the effects of their culture would be rather interesting. Possible 'lore' reason? Culture represents the strength of their 'signal' ie the reach of the Arch when it comes to controlling their ship avatars. While the signal can reach across several stellar systems, the actual culture itself is the 'optimum' range/area where they are at their most effective/have the shortest amount of control lag. Hence the rate of fire/building increases as they are more efficient. May or may not be possible as the actual culture, but could just as easily work as an ability of the culture centers. Which could be a titan spec hyperspace link or something . I don't know. The later might work better if you go with the former idea of a more defensive race, which means attacking them is harder.
As before, no fighers/bombers.. Of course, that leads to a bit of trouble when working out how to deal with the fighters of an aggressive/attacker. Which I think is fine on a small scale/when on the attack but not quite when on the offense. Anyway, I'm thinking that each defense should buff any other structure/ship in the gravity well slightly depending on the type. As well as system defense pods, I also had the idea that they should all be mobile, though that might be too much when combined with the pods/buffs. Basically the idea is a 'different type' of defensive race.' While their defenses are weaker individually then other races, once ports and networking are researched, they become the defense equivalent of the zergling rush.
Ralus: Anti Figher/Bomber defense. Has the typical autocannon/Laser equivalents, and drops tea pods (armed with the same weapon) 1-2 Logistic Slots, boosts ability to hit by 5%.
Malleus: Big badass anti capital/heavy defense, drops the system defense version of Double Expresso Pods. Once Titan/Seed AI tech is researched, could possibly mount one of the godhammer canons used by the battleship. 4-6- Logistic Slots. Boosts Range of things in the system by 5%
Panzer: Workhorse anti defense, targets frigates, drops Cappa pods, boosts RoF by 5%, costs 2-4 logistic slots.
Starbases: Two types, nicknamed the 'offensive' and the 'defensive'. Yes, I'm bad at names, didn't you get that from the pod types named ofter thte contents of my caffeine cupboard? Anyway, offensive one is mobile for sure, and is (you guessed it) based around attacking enemy planets, packing more gun upgrades then armor/shield. Special abilities are based around dropping large numbers of system defense type pods of various types. More focused on taking then holding, it packs a punch but lacks the anti culture/prevention abilities of the defensive star base. Grants a RoF upgrade for the networking basis.
Defensive bases are more based around holding the various things they take from the enemy. While they don't drop anything more dangerous then Iced coffa pods, and have more armor/shield upgrades then guns, the defensive star bases provide an upgradeable shield boost as a passive tech (through research). They also drop the Frappa repair pods, and possess the usual anti taking/culture and population upgrades.
Ship ideas are incoming, once I actually decide on them.