Yes, even if Mewtwo was made heavier, he'd still be floaty... A fast falling Mewtwo would look rather weird I think. Floatyness isn't all that bad, since it gives him good recovery and makes him harder to combo. I'd love to see Mewtwo having Samus' weight and floatyness combination, since that'd be perfect for him. Garb to combo, yet still very heavy.
I agree with Fawriel here that the design of Mewtwo's moves already is really awesome, and suits him really well. However I disagree with you that they need to be upgraded lightly... he needs a HUGE buff in his moves! His lag and lack of priority on his Aerials and Smashes is really bothersome, but he could use a buff in strenght and range aswell.
As for B moves, I love the Shadow Ball and Teleport, so they can stay. But his Down and Forward B need a complete redo, or perhaps even replacement moves. Shadow Ball already is stated to be a better move than Samus' Charge Shot, so I'm hoping it'll stay the same. Teleport works really well with Mewtwo's floatyness so therefore I hope he'll still be floaty.
For a Forward B I was thinking about Psy Beam, a rather short ranged energy wave projectile which has minor knockback but is very damaging and could act as an 'extanded grab'. The move would be executed by Mewtwo shooting the wave out of one of his hands, and could be slightly tilted. The move could possibly reflect weaker projectiles aswell, but they'd also be weakened a little in knockback and % unlike Fox and Falco's Reflector/Shine which also make the attack stronger. It now sounds like a terribly broken move, but the downside of it would be that Mewtwo can't spam this move (like Bowser's B) and if it has been used to the max (like when reflecting a stronger projectile or just after an extended throw or long use) it'll be toned down and act much like Confusion does now.
As for a Down B, I was thinking about Recover. Mewtwo sits down and meditate's shortly, after 4 seconds, he stands up and has a sprankling white aura around himself healing him slightly during he fights. The move acts a lot like a reverse Lip's Stick, and can has a healing maximum of 20%, Mewtwo can only use his move once in a stock though and it's interupted when Mewtwo gets hit by a normally fliching move while he's meditating. The best use of it would be after he'd KO'd the other character.
And to close this all down I say that Mewtwo deffinatly deserves to return and is still the most awesome Pokémon of all, and frankly the only one I really want. Screw Lucario, bring us Mewtwo back!