It wasn't too long back that Capcom Unity opened voting for most desired DLC characters in Marvel vs. Capcom 3 on their forums. It seems voting ended a bit earlier than previously reported, but the Capcom character thread alone saw over 6,000 responses in that time frame. Although there are no official tallies yet, fans keeping track of the votes have reported Mega Man X has been the top voted character, at least during the majority of the polling time. If not the most wanted, he's definitely up near the top. Other popular picks include classic Mega Man, Bass.EXE and Sigma. I should also point on that some months back, Japanese fans were polled with the same question, and in that case too, Mega Man X (ahem, Rockman X) was also the top Capcom pick. It would seem from the East to the West, this X guy is pretty dang popular.
Of course it goes without saying that this in no way gives any guarantee for Mega Man X appearing in later DLC. All it states is that the demand is there. Personally I've been finding Marvel vs. Capcom 3 to be incredibly fun as it is, and if anything Amaterasu is my top character so far. HHHHHHHHHHHH
Update: The official tallies are in, and Mega Man X ended up in second place with 2,061 votes, a scant 33 behind Strider Hiryu. Also making the top 20 is classic Mega Man, Bass.EXE, MegaMan.EXE and Sigma. A poll will be held on Unity at a later date allowing people to vote on the top 20 finalists in both the Capcom and Marvel sides. But again, this is not an official Capcom survey and will not determine any future DLC outcomes. Thanks, Aliltias!