Harebrained Schemes' Battletech has a robust selection of mech types as well as variants and customization options. Still, there are a few more designs I find myself hoping they'll add eventually such as the Raven, Javelin, and Cyclops. Then there is the Annihilator, the only other type of 100 ton mech to canonically exist in the Inner Sphere. Although, if you ask me, its design is more battleship than battlemech. Other than that, some more combined arms stuff like VTOLs and river gunboats might be cool. Aerospace fighters, while probably too complex to implement in their entirety, could still be used to vary mission dynamics by making the occasional battlefield strafing/bombing run.
One last thing, I want to mention is all the little nods this game has to older Battletech properties. I got a kick out of seeing the training simulators on-board the Argo (they look just like the machines used at Virtual World Entertainment Centers). The "power business suits" worn by independent merc contractors (complete with padded shoulders and brightly colored fabrics) are a nice reference to the 1980s influences on the visual style of Battletech. My favorite call back though, has to be the display readouts on in-game monitors. Whenever the player goes to check on their battlemechs or mechwarriors, direct copies of the record sheets used in the tabletop wargame and RPG can clearly be seen in the background. Great stuff!