Does the project builder support non-ez80 / non-ti targets too? just wondering.
Nope, it does not at the moment. However, modularity was one of the strong design goals, so that support for multiple targets - not just multiple toolchains for a given target, which is already in production for the TI-eZ80 native code module - can be added at some point.
Also, an idea might be integration to some kind of versioning like overleaf does. (You can edit LaTeX and host the project on github.)
Indeed. In fact, this particular item is already part of the wish list, alongside project import
The ability to export projects out of the PB infrastructure allows users to perform manual commits into whichever (D)SCM they see fit, but it's obviously more cumbersome than a direct integration into e.g. Github.