1: I know the thing says "aside from online", but separate absurd rules and competitive stuff from each other, like what Smash 4 did with "For Fun" and "For Glory". The latter can specifically only have Battlefield and evil FD/Omega as the stages with either 2-3 stock with 6-8 minutes respectively to suit the needs of various competitive rules. This way, people who want some competitive combat can play the second option, while absurd rules can go the other way while not affecting GSP... even though I still think this whole GSP thing is dumb from the start.
Someone also mentioned being able to change your character during Quickplay so you're not locked onto one character, and this could be helpful when you're fighting a character that's a bad matchup for you, but you're stuck at a disadvantage due to being unable to change characters without leaving. In the event that such a thing gets added in the future, there should be a 20-second timer to choose your next character after the match so it won't seem like they're stalling.
2: Better replay/snapshot functionality. I miss the times in Brawl where you could send replays and snapshots to friends, but now you can't. Just means more money's needed just so you can get vids record, though I like the times where you could send generous friends the replays to progress. Lots of money having to be spent just to get some of the stuff you need, not enjoyable.
There's also some stuff when taking snapshots that don't really work, like you can't scroll close to the end of a stage like you used to (Hyrule Temple is a good example of this), which is frustrating. Camera mode will also be disabled during certain things, like stage transformations from levels like the Kalos League stage or when that Fatal Frame girl appears. Other times you can't zoom in, example being when the Squid Sisters AT is on the field. Despite there being more features for snapshots in this game compared to the others, these various restrictions still hurt, especially as someone who likes to be creative with Smash snapshots.
3: Display the changes that each patch will have when they're announced. As some have complained about, when a patch is announced and then released, no one knows what changes are in effect, what characters have been buffed or nerfed, or any of that. With the previous patch, it only told us what characters were changed but not how. Patches should tell you what will be in effect and what's been added/removed so we don't have to keep labbing blindly. They should also announce bug fixes too if applicable.
Short version is:
1: Separate Competitive rules and absurd rules from each other while preventing GSP from affecting non-competitive rules.
2: Improve replay/snapshot functionality and enable the ability to send replays/snapshots to friends like Brawl did.
3: Patch changes should be listed when patches are announced and/or in effect.