And screwed up royally with Dominion Wars.
Things I would like to see in an add-on...
More playable races.= Tholians, Gorn, Species 8472, Cardassians, Dominion to name some ideas
More weapons. Phaser cannon & Disrupter cannon(heavy duty versions), Anti-Matter Spread(uh, TNG best of both worlds), Polaron Beams,
Missiles(bring them back), Plasmatic Pulsar Beam(sfc 2), Breen energy disrupter, for starters.
More mission diversity. Single player conquest is boring as hell. Multiplayer is alittle better, but still suffers from lack of diversity. Honestly, I'm sick and tired of planetary assaults, scans, convoy raids, and such. There's very little variation, its always the same thing.
Improved A.I. I'm not asking for genius here, but all the ai seem to do is come at me. They don't retreat, only flank me when I'm faced off against 3 or more, and don't see other ai as a threat.
Diverse Items. Not to many ideas on this one. Borg transporter inhibiters(can stop transport of a number of troops per level bought), able to select between fighters or shuttles. Multiple or better shuttle bays allowing faster launches, or multiple launches. Sensor decoys, Anti-cloak field generator, metaphasic shields(stronger against beam weapons, weaker against projectiles), Ablative armor(bought extra, adds to ship mass, and must be rebought to fix).