That's definitely a tough question, and the answer depends a lot on what you want out of the framework. With the exception of some examples, I generally have been keeping game related things out of MV3D itself. This is part of the planned direction of the framework, so it likely won't change. I envision someone (if not me) will write modules separate from the core MV3D code that include game logic, standard NPC stuff, etc; however, none of that is currently done or available. On the other hand though, I've been trying to get
Glyph excited about the prospect of MV3D &
Imaginary, which I think would be a great way to incorporate game logic into MV3D. I wouldn't be surprised to see that happen in the near future, but again, it's not in place now.
Back to the core of your question. Yes, I do believe that MV3D is stable enough for external developers. There are only a couple of changes I'm thinking of making that could effect external developers. One of those is moving some things over to being stored directly via Axiom. I'd hopefully be able to provide some sort of migration path there, but also, most of the stuff I'm moving over would only be an issue for you if you are planning to make your game isolated from other MV3D worlds (i.e. if you aren't part of the single sign on
described here).
I have definitely been looking to get more people to start using MV3D, so it would be great to have you on board. Bug report tickets would be excellent. Just let me know your Trac login and I'll give you permissions to make tickets and such. I think that the more people who are actively developing games or simulations using MV3D, the clearer picture I can get of what I need to concentrate on next.
Thanks,
Mike