I wonder if Fox would be allowed to keep their X-Men and Fantastic 4 rights if they bought Time Warner.
Surely there would be some contractual arrangement meaning they would have to relinquish the film rights to Marvel if they owned the rights to DC.
Doubtful for a couple of different reasons. One being regulatory, since Disney would be insane to not raise the issue of Fox owning DC along with certain Marvel film rights and the conflict of interest that would raise (to be fair, they likely already are/have prepared to do so once the initial offer became public). Another being that with the rights to DC (including merchandising), they have far more incentive to push DC over Marvel, who would then be a much more direct competitor.
Fox could easily resolve this by offering to sell the rights back to Marvel & Disney as both a regulatory concession, as well as apply said proceeds from the sale toward the entire acquisition of Time Warner. Similar to what Murdoch has said about selling CNN. From a transactional perspective, it's win-win.
They would have to part ways with X-Men and the revenue those films have generated, but IMO they could easily replace that with revenue from Justice League films. Plus on top of that they could produce films based on other DC properties. A better deal for Fox. Now whether they would do any DC film property justice remains to be seen.
That said, don't expect to hear anything from that regard until they actually strike a deal with Time Warner, as until then Fox has to continue to meet its contractual obligations to Marvel, and operate as if nothing is going to happen.
A very interesting potential turn of events for the world of comic book films if you ask me.