Yes, open sourcing at least the client part of the game is something that I'm very interested in. I'm not entirely sure when or how this will happen just yet, it will depend greatly on the commercial success of the game. But it's definitely my preferred course of action for the future.
EDIT: I'm not really interested in separating or spinning off the engine, but you will have few problems turning the game into something entirely different.
While I would advise against open sourcing the core of the game, opening the client api may be a good idea - perhaps others can write clients for it, but there may be issuing around people making bots to cheat the game.
I wrote a "bot" myself in the form of a network simulator for general and load testing of the server. If players were to automate the game I'd be more than happy to make a special server where we could show off their work. Making an agent navigate an open 3D space and performing planned tasks would make for an impressive demo for all parties involved.
That might make a good first open source release actually, if it implements a good chunk of the game protocol you might be able to use it to get a rough idea for the types of ideas people would want to run with, and it would also allow people to hit the ground running actually implementing ideas against the engine quickly.
A separate test server would be a very good companion idea.
I hadn't even considered this before, but I see your point! Getting a "core" part of the game's "API" released first allowing players to fiddle around writing a bot sounds like a beautiful gift for the technically inclined players. Because of the small(er) scope it could help iron out wrinkles in the open-sourcing of the client as a whole later on. Thanks!
EDIT: I'm not really interested in separating or spinning off the engine, but you will have few problems turning the game into something entirely different.