I for one am thrilled about Custom combos, but at the same time i hope they have few Combos in the Characters moveset just to give you and idea of what can be done.
So if they do do something like that, i think there should be Combo Challenge or a Practice Arena so you can learn some of there given combos before you venture out and start creating you own! As long as they arent ridiculously crazy hard im down for Combo challenge!