Missileboats would be nice. Some new types of missiles, as well. We have only have a few real Medium missiles. Most are just multi-fire versions of the small ones.
I'd really like to see a Torpedo Frigate of some sort, personally. Something with 4 bunched-together, forward-locked missile slots and only a single small energy or ballistic slot for PD. Front-locked, yet highly efficient shields and good speed but poor maneuverability. Make it that little frigate that ALL capital ships hate to see coming.
The problem is that it'd make it into a 1-shot-wonder. And generally that's the bigger problem with missile ships in general right now and probably why we don't see more of them; missiles run out fast and when they're gone, that's it. No reloading in the game (yet).