The differences between the two are pretty much why I'd want Akuma to be Ryu's echo. While his moveset would obviously be based on Ryu's, they could at least make him different enough to be interesting, and not an almost complete carbon copy like Lucina or Dark Pit (it helps that Chrom has established that Echo Fighters can have at least one different move than the character they're based on).

Also, some of the newcomers for Ultimate so far seem to be following a "villains/rivals" theme (Ridley, King K. Rool, and Dark Samus), and I think Akuma would fit that theme pretty well.