I'll apply this to my mains:
Wario- Wario's Fsmash goes the length as Wario Man's and the Bike has the same speed and power but be as easy to control as Yoshi's Eggroll.
Ness- Let PK fire burn like it can in Earthbound, let PK thunder paralyze like it can in Earthbound, and PK Flash can cause your opponent to cry uncontrollably, put them to sleep, or become KO'd instantly like in Earthbound.
Mario- Give FLUDD the stream length and strength that it has in Super Mario Sunshine. Also give it it's recovery properties like the jet and rocket. Also, the cape allows Mario to fly like in Super Mario World.
Ice Climbers- Blizzard premanently freezes your opponent.
King Dedede- Instead of Waddle Dees, he throws smart bombs and bom-ombs.
Fox- Fox's blaster has the power of Wolf's but still has the same range, speed, and spamability.