I still picture Matthew working as an alt costume, seeing as he's such a carbon copy of Isaac. You're replacing his clothes for more modern ones and changing his hair a little, they're also the same size, just about.
Felix though, he's always been taller than Isaac, I can't get that out of my mind. Having him in the game but the same height as Isaac would be weird to me, but not the worst thing ever. I'm sure they tweak the heights all the time to get characters in the game. That being said, if he's an echo fighter then I wonder if his height can be tweaked while keeping the model skeleton the same? Then make him heavier, and hit harder than Isaac to match his games.
Saying all that, i don't see an echo coming with the fighters pass, so it's really just wishful thinking.