- GURPS (Especially Transhuman Space)
- Warmaster (The recently-cancelled large scale 10mm fantasy/historical game)
- Full Metal Anorak (Especially but not exclusively Dirtside)
Honestly, I during the Clix years, there
was a BattleTech-shaped hole in the universe, as far as I was concerned. Dirtside was my replacement for BattleTech, and I still love it. The system is brilliantly clean and simple, and you have Stargrunt to "zoom in" for very small scenarios at 15mm. It's good even for playing in the BattleTech universe (its own universe is a little... skeletal, to but it charitably).
For RPG, I'd still pick GURPS over AToW, even for a battletech game. Transhuman Space is a great setting that has a unique feel that I greatly appreciate, and some of the best economics I've seen in an RPG. Eliminate the richness of 25 years of BT world-building, and I'd be playing THS exclusively.
GW cancelled Warmaster last week, but I still have the minis and love them (and there's significant historicals support for them as well from third parties). Haven't played in a long time, but presumably it would fill in some of the void.