Kailric
Jack of All Trades
In standard games, there is one player on each team. In custom games and multiplayer (multiplayer is always custom), you select team number of each player. Default is a unique team for each player.
Yep, In M:C however, I have added the ability to join teams with one Vassal player. They will be set to be on your team with all the benefits as you described. I am planning on improvements to the Vassal player.
@slow native research
Maybe we should make a "reduced research" trait, which we can give to the leaders we want to act "native". We can even make events or triggered events to remove such a trait if we like. We way also introduce techs, which can't be researched by certain civ types meaning natives will not evolve in certain directions without interacting with other civs. In fact we could make it impossible for a single civ to research everything themselves and trading/demanding/stealing techs from each other could be a major game factor. If we do that, then it should be a game option as it certainly is a matter of taste if that is a desired gameplay.
Yeah, I like the idea of the Barbarians slowly advancing on their own as well. Right now its still like vanilla in that they only advance as you give them weapons and armor in trade. Maybe they have a few techs they can research, and then you can sell them techs.. or your enemies can sell them techs to increase their power.

Making it so that some techs can't be researched the normal way is something I thought of too. We could have a few random techs that have secret means of acquiring, such as Greek Fire, or the famed Ulfberht swords. Such items could be built in your cities to become "Promotional Units" that simply have a special ability in that they can be consumed by your other Units to gain special promotions. There could even be a limit to the number you can produce. "Your blacksmith's have stumbled upon a unique mixture of iron alloy and can produce exceptional weapons for a time"
Good except you risk reverting your soldiers to no profession and lose all your weapons.
Well, they could be required to instantly return to the nearest city. That maybe the way I done it when professions are disallowed, I can't remember.