So well, I buy a ton of boosters. I waste a lot of these boosters because I most likely never will use Tower boosters. Currently at this point I have over 100 rank 1, over 25 rank 2, and over 7 rank 3 Tower boosters which will go to waste. I would much rather have someone else have use to these, if possible. :)

I just posted a suggestion similar to this, basically implementing a trade system in-game that would let you trade 5 or so of boost X for 1 of boost Y. So you could trade 5 tower cards for one Calvary or whatever. I have the same problem, I hate towers and never use them so I have tons of them just sitting around. I also got WAY more fort cards than I'm ever gonna need while I'm running low on everything else.

I like your idea too, for things like extra Calvary, Artillery, and Infantry, but it might be harder to do player-to-player card trades for things like extra forts because, honestly, who needs 70+ fort boosts? XD