I've always thought game would benefit from wider selection of crafts from the beginning. IMHO there should be no need of research to aquire improved interceptors. It would be better just to make them more expensive. I added Raven as Advanced Interceptor / Patrol Craft and Sentinel as Advanced Heavy Fighter.
I made stats like this:
Interceptor Raven Sentinel
Maximum Speed: 2,100 2500 2000
Acceleration: 3 4 4
Fuel Capacity: 1,000 1200 800
Weapon Pods: 2 2 2
Damage Capacity: 100 140 200
Initial/Monthly Fee: $500,000 $800,000 $900,000
It would be nice if Sentinel could have 3 weapon mounts instead of just 2, and Raven was only limited to light weapons (cannon, light missiles).
When it comes to later crafts I think it is too easy to be able to just use components from shot down ufo's, without the need of assembling them before further usage in Terran Crafts, ex.: navigation for Terran Crafts should be assembled from Elerium, Alien Alloys and Alien Navigation (same with Power Source). Also only then it could've been sold on market. IMO price of Alien UFO components should be 0 (as there is no possibility of sale restriction). Same goes for Alien weapons, but I imagine economy ballance doesn't fit this thread.