Fundamental changes I want to see, regardless of roster
>Multiplayer online smash run
>Customs refined for competitive viability (i.e. customs safe to use at tournaments)
>Inklings replace Miis in customization and playable archetypes
>Removal of rage
>Replace rage with a "super meter" that charges passively, kinda like limit but you can't control it. Once this super meter fills you straight up get your final smash, available to use. Smash Balls instantly fill the meter if items are on.
>Final smashes are balanced for more counterplay at higher levels because of this.
>(Thanks
Guybrush20X6
): Final Smash input being A + B to avoid accidental use
I don't think there is a way to balance Final Smashes without radically changing them and losing what makes them so special.
Now, I like the idea of a gauge, and there is a way to make it work in Smash Bros.
Street Fighter V has the V-Trigger, which is powered by a gauge that fills up throughout a match. One the meter is filled, the character gets access to a special ability, that can range from buffs to their attack, a special movemen option, or a unique attack they would not otherwise have access to.
Ryu, for example, has the Denji Renkai which charges his fists with electrical energy and grants him access to the Denji Hadouken and other buffs.
Ken has access to a dash that lets him close the gap between him and his opponent.
So, how would this work in Smash? Well, I have some ideas of what a V-Trigger system would look like for certain characters.
Mario(Fire Mario)
Mario's fists are surrounded by flames which give all his attack fire properties, increases his damage, and makes his Forward Smash more powerful with a larger hitbox.
Luigi(Thunderhand)
Luigi's Fists are surrounded with electricity, giving all his attacks electrical properties, increasing their damage, and changing his specials. His Fire Ball becomes an orb of electricity, and his Super Jump Punch gains a large hitbox and is much more powerful than normal. Luigi also gains buffs to his speed while in this state.
Bowser(Dry Bowser)
Using the magic of Kamek, Bowser transforms into his Dry Bowser form. In this state, he gains constant armor, with some attacks giving his Super Armor, his jumps are much higher, and he gains the ability to throw bones, which replace his Fire Breath attack.
Rosalina and Luma(Power of the Cosmos)
Rosalina gains access to a Barrier, that can block most attacks. She also gains the ability to shoot Star Bits at oppoents.
Donkey Kong(Coconut Shooter)
Donkey Kong pulls out the Coconut Shooter from DK 64. This weapon shoots powerful coconuts that do insane knockback and cannot be reflected.
Marth(Shield of Seals)
Marth wears the Shield of Seals on this arm, which is known as the Fire Emblem in his world. The Shield will reflect any projectile that hits it(barring ones that cannot normally be reflected) and Marth's counter is replaced with a parry with the shield, which causes the attack to stagger, leaving them open to a follow-up attack.
Ike(Urvan)
Ike replaces Ragnell with his father's axe Urvan. Urvan is much stronger then Ragnell, and can break shield easily as well as giving Ike more powerful attacks and smashes.
I could pretty much go on for most of the cast, but this is the idea I would go with. A gauge that gives all of the cast a buff that lasts until their gauge is drained.