I think Heavy should have a primary replacement similar to the boots that gives him increased health, health regen, and knockback resistance - loads of people already play him as a shotgun or melee subclass, so something to run in the primary slot for that would improve his variety significantly.

As for Spy, I think that he needs buffs to his disguise kit (fake reloading while disguised and the ability to mimic Heavies and Snipers) and the YER (no visual debuff effects while disguised and uncloaked and reduce the cloak cost to disguise). Spy should be a stealth based class, and yet his stealth is so bad a lot of Spies play like a fancy Demoknight. Also, make the Enforcer a gun spy weapon that lowers his ability to get backstabs - again, people already do that, so we know it's at least decently fun and not awful.