Malte,
I am not sure I am following you :)
The templates I am referrign to would be for content display. I haven't thought about how to associate content with multiple subsites, but I suspect the built in CR template mapping function can be used. It has a context which can be any abitratry string. One way to use that would be to specify a different content template for content in different subsites.
The forms would be strictly for content editing. It might not even be too important if we have a fairly comprehensive set of default content types.
