If you (and/or your players) are a fan of the movie Top Gun, you could do a twist on the academy cadets idea. (or perhaps a follow-on campaign idea..)
The Kensai Kami is House Kurita's "Top Gun" program for mechwarriors. Few details have actually ever been published about it, so feel free to just revisit the plot of the movie :D
If you're not keen on the flavor of House Kurita, you could set your game during the 2nd Star League or a period of detente with the Federated Commonwealth.. foreign exchange pilots would have been plausible. Or alternately, any of the militaries might have a similar program.