Keep in mind that Fox doesn't "own" anything. Marvel licensed them rights to use under very specific circumstances. The only thing preventing Marvel from developing a Fantastic Four or X-Men film right now is the possibility that Fox could sue them for violating their licensing agreement.
Realistically, nobody is going to sue anybody now, but Marvel management has probably instructed Feige to not develop any properties for two reasons:
1. On the off chance this all falls through, there's no point in wasting effort on projects they might not be able to use.
2. Marvel wouldn't want the appearance of any impropriety since the two entities need to function independently right up until the moment the deal closes.
I'm as certain as I could be that the moment the deal closes, Feige will be able to jump in with both feet. The only thing that would stop him would be an edict from upper management telling him to hold off for some reason.
And I can't imagine any logical reason for that edict, so I assume they're going to turn him loose.