You know what would be nice? Annother hotkey, like "e", but for context-sensitive help calls. It would be nice to be an engineer working on a sentry, see a soldier coming up, and be able to target him and press, for instance, "q" (though it may already be used), and say "Fellas! There's a Soldier commin' at me!" and maybe give everyone else a little icon similar to the Medic's "Heal Me!" balloon. You might also be able to target an ally, though I'm not sure if it should have you say "I'm on my way to help you" or "You in particular, help me!". I'd lean towards the first, since beggar's can't be choosers.
It has a number of fringe benefits. For one, it would be about a zillion new voice recordings and interesting ways for the characters to interact, and two, it would expand the way the spy can trick medics to apply to everyone, which opens up the class- though at the same time, he is now more vulnerable to people spotting him and yelling HELP at him, causing everyone on the team to know they are there.
Heavy (general, healthy): Comrades! To me!
Heavy (enemy scout, healthy): Help catch tiny worm for crushing!
Heavy (allied pyro, healthy): I weill help funny little rubber person!
Heavy (enemy scout, injured): Help me! Tiny Baby is ANGRY!
Engineer (allied, damaged sentry, healthy): I'll fix that sentry for ya, pardner.
Pyro (Any enemy, healthy) [confident muffling]
Pyro (Any enemy, injured) [frantic muffling]
Medic (Allied Heavy, healthy): Vot an exelvent specimen! I shall vork vith you!
Medic (Ally, healthy, ubercharge ready): Ve are ready for ze charge, yes?
Ok, the pyro's pretty easy to write for.