Looking at the amount of newcomers we have now, a harsh truth has hit me:
Bringing back veterans is much, much harder than it looks. Especially those from Melee and Brawl not in 4.
How do you make absolutely sure they can't do anything abusive with their grabs, yet also make them good enough to not need it?
How do you make Pichu not completely awful with his handicap?
Look at him! Other than his specials his moves and animations are barely recognizable from his Brawl iteration.
Those are just the standout examples. They all require significant tweaks to function in the new Ultimate environment, to say nothing of balancing. It's no wonder there wasn't much resources to go around making a lot of brand new fighters.