I never played D1, but Combined Arms looks like it would fit perfectly in D2. Of course, we would need new maps, but here are some things I think would be cool:
-Obviously, there are Drakes and Pikes and such
-Defeating enemies gives you some sort of token that you could turn in for AI allies;
-1 for a Rasputin Sweeper bot
-5 for a Heavy Frame
-10 for an Elite Heavy Frame
-Heavy frames would use champion modifiers (Barrier, Overload etc.)
-Getting an opposing Guardian down to under 50 health would let you use a finisher on them
-Increased Heavy spawns
-Depending on the map, you could turn in tokens for things like Hive Cleavers, Scorch Cannons, Valkyries, etc.
-Supers would have increased duration (and one shot supers like Nova Bomb would do much more damage).
These are just some ideas I think would be cool. It would be awesome to see a Drake duking it out with a Heavy Frame.