I just had a thought:
How about making units that are built on the campaign map elite versions of their base unit type for the purpose of RTS battles? By elite troops I mean higher stats overall for those starting units.
It was just a sudden thought, so I'm not sure how good an idea it is.
I think that'd only make RTS battles more redundant as your better units would just roll over all the regular ones...
But I like the idea of a veterancy system. Perhaps the more battles your units on your campaign maps participate in, they gain increased veterancy and their stats are improved that way?