Alby_da_Wolf on 15 March 2014
The most recent alternative, considering only mech games, is Hawken, that is MP-only too, but it's free to play.
Alternatives with large SP campaigns are almost all quite old or even very old.
http://www.gameguru.in/features/2013/08/best-mech-games/
http://www.gameranx.com/features/id/15934/article/best-mech-games/
