Carriers in the traditional sense simply are not needed in the galciv universe. Here are, in my opinion, a number of things that would need to be changed for carriers to make sense.
1) Logistics. That is, reloads and refueling. In this game, the logistical aspects of keeping a ship functioning are done automatically, and you can extend how far a ship can travel from a base by adding life support (or just researching the life support techs). If ships had to occasionally go to a space station or planet just to get ammunition reloads and fuel, then carriers would make sense.
2) Fleet Management. Because of the way fleets work, having smaller ships means you can have more in a fleet than if you were using larger ships. Carriers, as commonly presented, would be a way to 'cheat,' either granting you a larger fleet than you should normally be able to have, or by allowing you to make small ships without any real engines, and thus be more powerful than the current game balance expects, which ties into the next thing.
3) Speed. There is a trade-off between speed and armaments. We all know how worthless a single-engine ship is. Carriers would unbalance things by allowing you to have tons of speed on the carrier, and not having to worry about it for the carried ships.
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That being said, there are ways that a 'carrier' class ship could be usefull, even though they wouldn't be a carrier in the traditional sense.
I think the largest disapointment in this game would be military starbases. They are expensive, fragile, and just not that usefull in a decent-sized galaxy. With the amount of resources spent getting a built-up military starbase near an enemy right before you attack, you could probably have just built more attack ships and ended up with the same, or more favorable results, except in extreme situations (200+ ships in a stack, for example). Here is where it ties into the carrier discussion: Carriers are designed to be able to increase the amount of firepower you fleet can field, without appreciably increasing the size of the fleet. The do this by fielding small-but-powerful ships that couldn't make it to the fight on their own (a concept which I feel is not needed in this game). Instead of trying to create something entirely new, why not just use the game mechanic already in place to increase the effectiveness of a fleet -- make Carrier-type ships that fields a variety of modules which increase the effectiveness of nearby ships (by nearby, I mean "in the fleet with them"), much like a military starbase. Obviously, the starbase should be able to have signifigantly more of those modules, since it is stationary.
Doing this would have pretty much the same result of adding more conventional carriers. People would probably field fleets of smaller ships and re-enforce those fleets with a few very expensive carriers, thus increasing the power of those small ships immensely.
A few points: Carrier modules should be huge. So big, in fact, that they can't even be placed on anything smaller than a Medium hull, and if you want more than just the basic modules, you have to use large or huge ships. Also, adding the military modules should increase the amount of logistics points that ship uses.
The size of the modules would mean that, yes, cargo hulls could field those modules, but because of the way the combat engine is designed, it would be foolish to do that, since cargo hulls would always be attacked first, and destroyed on the first hit that did damage.
