I haven't played for hundreds of hours (yet), but I just got murdered, saw my opponent grabbing the Smash Ball and a Blue Shell while I respawned, and I got murdered right again, so here are my unsolicited opinions about improving Ultimate:
1. There should be more mobility when you respawn, like flying around in a UFO for a bit and then jumping out of it or something like that.
2. Item spawning should be balanced around character speed and position. For the basic idea, picture a flat stage with two characters standing there. A single item can spawn in the middle between the characters, meaning when both characters start running towards it simultaneously, the faster one will grab it. A more fair reference point for spawning is wherever both characters would meet when they started running towards each other simultaneously. Of course, items only ever spawning exactly at that reference point would be pretty dull. Instead, this could be used together with some randomization and a tracker checking whether there is a strong trend of being particularly fair or unfair towards a player in how the items have spawned and this potentially triggering a different spawn point to make up for that. You also need to take verticality into account, but that's the idea basically. (Perhaps something like that is already in the game; if so it's too subtle for me to notice.)
3. The Final Smash Meter as an alternative to the Smash Ball seems like a really half-baked idea. I think it should take longer to get the F Smash from this (especially with characters that have a very strong F Smash) and when the bar is full, I think it would be more interesting if the character pulsated in and out of the state where you can do the F Smash, like 1 second in, 2 seconds out, 1 second in or something like that.
4. Attacks by slow characters should have more hitstun.
5. There should be an automatic slow-mo replay of the kills after a fight because I often have no idea what just happened. I feel like I'm Joe Biden driving a car. Where am I? Who just got killed and how? When somebody got killed by stage hazard or an item, a little line of text explaining what it was would be helpful. Oh my God, I really sound like I'm getting senile :(