Interesting idea and kinda cool.
I think the Soldier should be able to use the throwable grenades from his ammo belt. I really think the soldier could benefit from that.
Also, while we're on explosives... for the Spy update I would like to see an alternative to the sapper, where the Spy can place some C4 explosives onto sentries etc.

I like this C4 idea. Would it be timed or remote triggered?
It would be funny if an Engineer runs back to difuse what he thinks is a Sapper then an invisible Spy nearby (I think he should be able to detonate whilst cloaked) blows him up just as he gets there. I don't think it should take off all his life but if he'd just been in a firefight and was running back to save his Sentry then his health might be depleted enough to die from a C4 explosion.
Plus it could be used on dispensers to take out troops who are falling back to heal themself. If a Soldier and a Demo ran up with low health and the Spy detonated it could take them out. Of course they'd have to be blind not to see it but let's face it, when you're on fire you tend to miss a lot of things.
I'm really digging this idea Jinja. Nice one mate.