they could, but i don't think the character has the following they'd want in an Iron Man replacement... i love Don Cheadle, but he hasn't done much and with the average Marvel film fan loving RDJ as Iron Man and without question viewing Iron Patriot/Don Cheadle as a direct replacement, there would be a major drop off
fox and marvel studios have traded some franchises already (Daredevil, Ghost Rider)... but i don't see those larger franchises moving without big paydays going to Disney. of course, with Lucas Films/Star Wars/Indiana Jones all being scooped up in the very recent past, it wouldn't be out of the question
in my personal opinion... fox can hold on to F4 and drag them through the mud all they want. never been a huge fan and can't see them fitting into the Marvel U that Marvel Studios has created... too happy go-lucky and not real world enough.
Them getting x men though would be insane. Sooo many storylines could be played out