First off, I have no issues with the way the Frigate, Cruiser, Captial ship units are in-game. I just feel like there should be more of a variety. For example, I'd like to see Frigates, Destroyers, Crusiers, Battleships, and Capital Ships. 

I wouldnt change a thing with the current frigates, and cruisers, but just add some destroyer and battleship units. Currently Frigates and Cruisers are pretty specialized. Your destroyers could be more of just direct combat with other ships with bonuses vs Frigates to help counter Frigate swarms. Basically have two destroyers, one with guns only and some light point defense. And have the other destroyer blended with guns and missiles. Or a Guided Missile destroyer with a couple gun turrets. They could serve as fleet escorts to provide a blended defense against enemy ships and strikecraft. Of coure the LRM Cruiser and Flak Frigate would be stronger at both those roles. 

Battleships could serve as Cruiser/Capital Ship deterents and as system assualt ships to last longer in fights with heavily defended systems. Longer range guns, heavy armor, point defense as well. Offensively a little stronger than a Heavy Cruiser, but more armor. Point defense as well on board. 

On the tech tree, only feedback currently is to provide additional late game options to continue/improve weapons, armor, antimatter etc.... and it could just continue on as enhancements in the tree. Gives us something to spend our resources on in very late game.