Design changes:
Universally the difference between slow and fast attacks needs to be less significant. What happens is as players get better, a lot of these moves become obscure punishes/mindgames you see maybe once every other match. This already seems to be the case with Bowser, Link and Samus, so I hope this carries onward.
Overall less floatiness, so that characters who are actually meant to be floaty have appropriate strengths and weaknesses.
Ganondorf to still have the same brute character design, but make use of either his sword or trident in addition to stronger punches and kicks, and granted Dead Man's Volley.
Ike to be replaced with Chrom. Different technique but same general character direction and attack properties. Given his own B and B forward specials that are actually interesting. A non-spammy javelin would give him a bit more stage control.
Specifics:
Samus to be given missiles that actually kill again.
Link to be faster overall but retain stiffness and poor recovery. Return of SSB64 boomerang, giving it more KO power.
Jigglypuff removed and aerial prowess granted to Kirby.
Mother series characters to use a Pit-esque reinterpretation suitable for a 3d environemnt, making them a bit taller and giving them much needed reach.
Less priority given to Meta Knight attacks that are just not fun to fight against. :/