Like others said, I don't think the game really needs more ship types/sizes, what it really needs is more variety and options with current existing ships. New weapons and components that add different options, new ship sections for more variety/customization, and most importantly more Combat AI options to custom tailor a ships AI, rather than just having each size run the same AI.
Of course, this all sounds like something for a military-focused patch/DLC. I imagine such a patch would also address a solution to doomstacks, defense stations being useless, warscore and war strategy, ground combat enhancements, and who knows what else. Likely to be either the 2nd or 3rd expansion I'd bet (the other one being Trade and Diplomacy).