If they make a DMC4 special edition(which they might not but a boy can dream) they should have new artwork, new bonus features, a audio room(like the HD collection), & all that obvious easy stuff Capcom can cram in but what should be in there is new modes like Boss Battle Modes & Boss Survival Modes, an improved Bloody Palace Mode(almost like DMC3) or is that too much, a few extra missions, slightly altered story with extra cutscenes, new enemies, actual costumes(not just Super Costumes) that even appears in story cutscenes, a few new bosses(make Nero a boss), & most importantly the option to select Nero or Dante for ANY mission(once completing the game once).
it all depends on how much capcom does to fix the mistakes of both which are many but than i rather see a dmc5 tho.
potentially they would have to remake the game from the ground up which might be a waste of resources but since the series has no real direction, you could afford to that.