I think they should change Luigi's movements up a little. 'Cause, as of late, Luigi's been faster and jumps higher (and, as such, farther) but weaker and lighter than Mario and, of course, has lower traction.. So...he should have more horizontal movement in the air and not just for his Up B (which I think should go higher as well...<_<) but for his jumps as well. Maybe make him play like a heavier floater without the floating.
He needs an actually useful projectile because defying gravity isn't a good balance for being slower and weaker. If he keeps his fireball it should be faster and stronger (and should make people burn green...<_<) but if they go with something like the Thunderhand...I just want him to have a useful projectile.
With more horizontal-ness, the Missile could be obsolete...I always thought it was way to laggy and his recovery setup altogether really held him back somewhat. If it was faster, like they seemingly made Pikachu's Skullbash, I wouldn't mind it back.
His Super Jump Punch needs some horizontal movement to it, like most people have said, but I think it should be higher but not as horizontal as Mario's. Like, maybe a little closer to Mr. Game & Watch's in arc. I think it'd be cool if the window for the sweet-spot re-opened at the peak of the attack...but maybe that's just wishful thinking.
The Cyclone was a rather cool move. Luigi's variant could remain as his down-b to keep intact everything about it from SSBM...I'd rather not lose his down aerial move. :/
As for his standard attacks, more "Luigification" would be cool...but remaining alike to Mario would still make just as much sense.
Also, in regards to his vacuum...I don't really care either way. His neutral B or forward B could be replaced (as long as the other changes above occur) and really care...Being used in his Final Smash would be pretty cool. Maybe use the emblems for a Mario Finale type attack or one of the cooler ones that have been suggested in this thread.
Regardless, I'll still play as Luigi 'cause he's Luigi. :/ C'mon Luigi update. :O