I would add Chun Li first since she's the Queen of SF and focuses on kicking while Ken will mostly have a similar move set to Ryu's, personally.

Truthfully, I would LOVE if it was just a HD remaster/fine tuning of Melee, with none of the new or additional characters, stage or items from the sequels (except maybe as DLC only if they can be balanced to fit its style).

If not that then all the 3DS stages, Smash run and a return to a classic Adventure mode or something similar. Really, I don't see a Smash for NX if it is a launch title being a true sequel and it will probably be the first game in the series Sakurai isn't directly handling so they can experiment with a port/remaster here and see who or what might be able to carry the franchise into the future besides changing the roster.