@Buyable upgrades; I really hope, if such a system ever is put in place, it's only per-round. Having rank-unlocks outside per-profile would nearly completely destroy my interest in the game. It's one of the things that's so shitty about most modern shooters.
@Building damage; That sounds like a very good idea, but I imagine it being really hard to balance around marathon and non-marathon.
You really shouldn't state your opinion as if it's an universal law. You simply can not know how many people didn't keep playing just because the game has mines.
Anyway, removing mines would completely cripple GDI and make the game unplayable. GDI relies on having them as a steady defense against SBHs.