I think it's best if you tell everyone what you want to do exactly... that way it's easier to provide a solution.
i want to be able to create a bunch of different neutral/tileset objects. for example different houses and decorative stuff.
but i definitly don't want them to be randomly placed, because then i never know, how it looks in the end.
so far, there are exactly 10 different neutral objects possible with the editor. in warcraft 3, i can have dozends if not hundrets. that helps a lot to create maps/scenarios with athmosphere.
a simple way to create own neutral objects would be to just use units of a extra faction. but then, it had to be possible to make them invulnerable.
about the team discussion: even if it was possible to make the neutral player everybodys teamate, you could still attack its units. that wouldn't be to nice, for exaple if there is a big custom rock. you could just destroy it. that should not be possible. so for this problem, it would still be no full solution.