Let's not use melee as a standard. It was fun, but it was also broken as all hell.
I would have preferred that Lucina, Marth, and Chrom all been alt skins for each other because Chrom and Lucina literally have the same fighting style, so really the differences would only be nuances in speed and power. And besides, while Chom would be stronger, I don't actually see him as an actual rival to Ike in pure strength.
Isaac. It has to happen now. The fans have spoken. AND HE HAS SO MUCH POTENTIAL AS A FIGHTER. There are so many ways to approach him as a character. He could have a move like shulk- he sets different djinni, which would boost his speed and knockback (jupiter/wind), attack and burn damage (mars/fire), defense and super armor (venus/earth), healing/slows/water pushes (mercury/water). His specials and smashes could be based on weapon unleashes or various attacking/field psynergy. His final smash could be any number of awesome attacks- Odyssey psynergy, Megiddo (sol blade unleash), Judgement (summon), Grand Gaia psynergy, Titan Blade unleash... Isaac can be so much fun if they really explore all the possibilities.