Starting in Super Mario Bros. The Lost Worlds Luigi could jump much higher but was more slippery. So maybe you could make his jumps higher and more slippery.
In Super Mario World Luigi had a flutter jump or basically a lot less weight.
In Super Mario 64 DS Luigi was slower but again, jumps high but the second weakest in the game.
In Luigi's Mansion he uses the Poltergust 3000 and that could be used for a lot of moves.
It can just normally vacuum which could be used kind of like the opposite of mario's FLUDD to pull Enemy's toward you. It can also shoot fire to shoot fire and that could be used like bowsers normal special. It could shoot water, kinda like FLUDD. It can also shoot ice which could be used to freeze people like a Freezie.
In Luigi's Mansion 2: Dark Moon Luigi uses the Poltergust 5000 and that is much better than the 3000. Here is a little paragraph from a mario wiki thing about one of it's feature's
I think that could definitely be used while "Sucking Up" enemy's and if you let go of the button between certain intervals it adds different amounts of percentage. It also has a flashlight that after being charged up (he moves slower while charging up) flashes and stuns enemies for a second or two. One more thing it can do is called the "Darklight" and it, in game reveals things that are not able to be seen or something. I'm not really sure how this could be used though.
In Mario And Luigi: Superstar Saga Luigi can use his Thunderhand ability to electrocute enemies. That would be a cool Charge Attack.
In Mario & Luigi: Bowser's Inside Story One of the Bros moves luigi eats a lot of food and grows super fat and gets launched and lands on the enemys and do massive damage. I was thinking this could make a great Final Smash if you could make those.