There's a LOT Smash could stand to learn from other platform fighters, but for the sake of this post, I'll just stick with the basics:
1. Universal movement mechanics
Whether it's airdashing or wavedashing or whatever, I think it's telling that every non-Smash platform fighter on the market has some sort of movement tech Smash doesn't. This is much better since not only does it mean even the sluggish low-tiers are able to compete, but it also enables the designers to get more creative with more powerful movesets since every character has the means to play around them, thus making the game more fun for everyone. The worst characters in Smash like Ganondorf and Dedede have no movement tech whatsoever and are extremely slow, leaving them almost unplayably vulnerable to certain styles of play (i.e. camping).
2. Multiple attack buttons
This one's a given. One face button should be for light attacks, another should be for heavy attacks, and specials can stay on their own, separate button, with the fourth usually going to jump. This is an objectively superior system that makes inputting the attack you want on the fly significantly easier, which is why every single platform fighter bar Smash is doing it.
3. Better counterplay to projectiles
This one's kind of lumped in with the first one, but most platform fighters handle the whole projectile thing way better than Smash does. Usually, this comes in the form of some sort of universal reflector like Rivals of Aether's parry or strong attacks in NASB. In Smash, if your character doesn't have a reflector baked into their moveset, approaching someone who's insistent on camping with projectiles is just kind of a slog and makes countering defensive play boring at best and outright agonizing at worst.
4. Dedicated ranked and casual matchmaking
This is an incredibly basic one that not only every platform fighter, but nearly every multiplayer game besides Smash is doing. Separate matchmaking pools for people who wanna play for rank VS. those who just wanna get some games in is an extremely obvious call since those people aren't going to want to play with each other in any universe. Also, screw battle arenas! Just bring back friend rooms! They were an extremely convenient system that did practically everything arenas do better!
5. Cool it with the stages, please
I have never once met someone who wanted to play on Pac-Land, or Great Cave Offensive, or Palutena's Temple. There's a difference between a stage being designed for casuals and being unconducive to fun gameplay. Casual stages, in my opinion, should be more like Mushroom Kingdom U and Kalos Pokemon League, having unique, yet not exploitable layouts and fair, predictable hazards that don't leave you feeling like you're fighting the stage more than the other players.
That's all for now.