I'll repeat that it's really not a good design direction. Okay we could tweak parameters on them, but just imagine tring to make them really balanced. Perhaps I should go about this in a different way. Final smashes can be broken into three rough catagories.
Transformation (Bowser, Diddy, Yoshi, Wario, R.O.B., Fox, Falco, Wolf, Pikachu, Mr. Game & Watch, Sonic): Character becomes something broken and the opponent(s) have no choice but to run from it. In practice this usually means a guaranteed stock loss (Falco has the best FS because he can very plausibly take two stocks!) though some of the worst transformations (G&W, Diddy) are more frequently survivable.
Big hit (Mario, Link, Zelda, Sheik, Ganondorf, Toon Link, Samus, Zero Suit Samus, Ice Climbers, Kirby, Meta Knight, Captain Falcon, Pokemon Trainer, Marth, Ike): These tend to kill if they hit except at very low percentages and are usually pretty easy to land though how easy varies wildly. Zelda & Sheik have the best one of these (Kirby's is close) and Pokemon Trainer's is clearly the worst one like this since it's super easy to SDI out and not die and I dunno why his is so bad compared to the others. Either way, having a big hit final smash is basically just plain worse than a transformation since it's "you get an instant kill if you do it right" instead of just "you get an instant kill" but still not bad.
Gimmick (Luigi, Peach, Donkey Kong, Pit, King Dedede, Olimar, Lucario, Jigglypuff, Ness, Lucas, Snake): These are pretty universally worse than the other two types. A few like DDD's and Olimar's still represent good damage, a few like DK's are just really situational which is ill-fitting a final smash to say the least, and a handful are actually bad moves just as moves not even as final smashes (Ness, Lucas, Snake should never hit in singles with their final smashes and will be punished for trying).
So not only are these three catagories pretty ridiculous to try to balance since just fundamentally they're of different value, but the gameplay isn't even that good. If you get a smash ball as a transformer, just move close to the enemy, transform, and then go to town. It's almost a sure thing you take a stock, and as Falco if you do things right you have a real shot at taking two stocks (Falco's final smash is the best Landmaster and therefore the best final smash in the game). We could make the going to town part less deadly, but the basic gameplay of "I'm chasing you with impending doom while you run away waiting for the timer to run out" isn't very compelling. Due to how they are programmed, substnatial nerfs to Warioman and Giga Bowser would be at least as much work as designing an entire character each. The big hits are just basically fishing for a single hit that offers a ridiculous reward. It's certainly more interesting than the transformations, but the whole value metric of these moves is similarity to instant kills. Weakening them substantially would maybe be a workable metric if every final smash were in this archetype, but that's not the case. Then the gimmicks are mostly just a lost cause, especially some of the most poorly built ones like Snake's or Peach's that are just fundamentally bad ideas outside of FFA. Most of the gimmicks aren't exactly fun or interesting either...
I know it's a shame that final smashes look so cool and were obviously such a big thing in development yet aren't used, but they really aren't balancable in a reasonable way, and it is clear that the cast was really balanced outside of them in standard Brawl and has been honed in Balanced Brawl still not considering them. They also frequently don't work very well on a variety of stages or across game modes (like singles vs doubles) which is an extra headache, but even just considering singles I'm pretty convinced it's impossible to improve the game by including them and almost impossible to avoid hurting the game by doing so.
