Hello Jan,
I’m still interested in helping you by building a service based on OSM2World. As you know an early prototype already exists and I could probably finish the project according to your final requirements.
What I cannot promise yet is whether I’d manage to provide worldwide coverage in the short term, but I’d be willing to at least try whether it works out.
To clarify, are you hoping to have this service hosted for you by someone else? If so, what kind of load is to be expected? This is probably going to be the main obstacle if you choose OSM2World: I’m unlikely to have access to more powerful server resources than you have available yourself.
I do have another suggestion, though, even if this means that I’m advertising an alternative solution from what I can offer myself: Have you looked into Overpass API? It does not have any building-specific intelligence, but is instead a general-purpose API. If you are willing to do the rest on the client side, though (and I remember you were already experimenting with that for the simple shapes?), it could easily give you all building polygons in an area. Availability and stability are pretty good, too.
We could even combine the approaches: Get all buildings where you have client code for rendering from Overpass, and fall back on a backend based on OSM2World or one of the other 3D renderers or the more “interesting” buildings only. This might make it a lot easier to achieve worldwide coverage.