Titans of Steel is free now. What about a ToME-net styled GH MUD/RLMMORPG (Roguelike MMORPG)? I'd host a server, heck, I'd buy my own server to host a server. Maybe. If I have enough cash. Or a sufficiently cheap server.
Anyways, the RTS would work very well, given that Total Annihilation, which Spring emulates, is primarily mech based, with additional unit types. It supports units on different scales (SF:1, SF:2, even SF:3, given a large enough map.