I think there’s a good chance that he will appear in SfV. Think about it, both Guy and Cody appeared in SF4 but Mike Haggar only appeared in very limited number of fighting games. In fact, Mike Haggar is in USF4 but only in the background (as well as Sodom). So far most of street fighter characters who appeared in the background showed up in the game. Come on capcom.
I think long time ago someone started a petition for Mike haggar, not sure what happened to it