"Tom and I have discussed the idea of removing global state from ArjunaCore and it looks achievable. We don't envisage a scenario where you wed yourself to ArjunaCore and then we say that we can't remove the shared state. Mark also didn't seem to think this would be a problem, although take that as idle gossip until he confirms or denys ;-)"
It shouldn't be a problem, but I'd have to look through the code to be sure. It's not going to be as simple as changes isolated to ArjunaCore though, because potentially everything that layers on it may be impacted by the changes, cf statistics gathering. However, any changes should not be visible through interfaces and classes directly.