Mario's is most likely based on his strongest fire attack from SMRPG.
Luigi's is based on the fact that he's always overshadowed and overlooked. I wouldn't change either personally.
By that reasoning, there are several other FSes that should be changed as well.
Really, I think Mario's FS is perfectly fine, as it's just a natural extension of what he does do in his games (fire flower), and the only real alternative is already a normal item in the game (mushroom) or is taken by other characters (mega-size).
As for Luigi, while it makes no sense at all, I think that's kind of the point behind it. However, I have no problem with it changing, if others want it out.
EDIT: What SGG just said...
If anybody does think that it should be changed, I have an Idea for Mario'.
It could be that fire move he uses from SMRPG, where he throws a bunch of gicantic fireballs at other players. Or maybe he could use his hammer for something.
I think Mario's final smash is heavily influenced by that SMRPG attack already, they just altered a bit to make it more aesthetically pleasing.
Luigi could use with a better final smash though, he's current one is pretty lame x.x; What's one thing he's got that nobody else does, though?
He could use his thunder attacks from Superstar Saga.
We've gone through this. No. He's used fire way more than thunder. Fireballs are way more iconic.
We've gone over it for his final smash too? Nevermind then.
Why not base his final smash after the Ultimate kick from Mario strikers (the soccer game).
However I forget what that is.
His F smash was to have thunder effects correct?
Luigi's FS is ironically more effective than Mario's. 90% of the time you can leap over Mario's attack.
There is no need to change Luigi's final smash. Changing a clone character to be more original? Sure, thats fine, but why change one of the most original FS's in the game?
well Luigi could pull out his poltergust and could walk around the stage sucking up characters and launching them helplessly to there deaths this is a pretty original idea and it reps him better and for mario i think his is fine it seems more him anyway
See, if anything we could just do to Luigi what they did to Mario. Take his down B and make it his down A, then make his down B the Poltergust, or better yet, just make him use it in his grab or something.
Luigi's Negative Zone is essentially a combined showing of all the abuse he's gone through in the series, and seeing as Luigi is pretty much the Mario worlds punching bag, I think that his current final smash is the only one that is truly fitting for him in terms of representing who the character is.
If it can't be Luigi's final smash we could weaken it and turn it into an ordinary item. (it would work a bit like the firecracker)
I keep thinking that we've already decided on putting the Poltergust somewhere in Luigi's moveset, haha.
It might work well as his Down B move, (pulling people towards him, rather than pushing them away like FLUDD) but we could also use it to give his grabs more range. Maybe if a character is a bit too far away for him to grab, he automatically takes out the poltergust and pulls them closer, then goes into the grab move? Orrrr he could use it to repel items thrown at him or certain types of projectiles (it had the ability to absorb fire, ice and water and throw them back out as an attack in Luigi's Mansion, iirc)