So who else really needs buffs? I keep hearing people mention how Ganondorf really needs them, but he's seem alright to me. It would be nice if he got some though.
Ganondorf's often cited as a perfect example of "Good in more casual settings, but horribly exploited by people who really know his weaknesses". The latter's precisely why a lot of more competitive people have argued for him to get buffs / a rework: Sure,
a Ganondorf who knows what he's doing is a terrifying opponent who can erase lives in the blink of an eye* (and I believe that's why Sakurai's kept him that way), but he's also practically a sitting duck when he's in a bad spot (Ganondorf's infamous for being combo food, for example). The fact that he's been saddled with many over-the-top but slow moves (Warlock Punch, Volcano Kick) doesn't help because he loses versatility for
the best attacks in the game BIG moves.
Another issue many more casual fans have with Ganondorf's moveset is that it's still showing the scars of his first appearance as a Falcon clone and is thusly not the King of Evil. A sentiment some more competitive people like MockRock have also expressed. One argument from this camp is "Why keep Warlock Punch when Ganondorf's had so many iconic attacks to him?" That's another group that primarily want to see a rework - especially those that want to include the original Ganon incarnation.
Absent a complete rework (honestly I'd like to see at least some of Ganondorf's moves changed at this point): what I think would help him is if he got his float (like in Project M). It should be somewhat technical / limited so it's not used willy-nilly, but the float would help his recovery some. Or: letting his Wizard's Foot go through some more projectiles, since would help his approach a bit (even if Wizard's Foot is fast, it's commital too).
And as for others that could use buffs / reworks... I'll give three examples:

: Noble experiment, but "You ain't an Air Fighter!" has been so pidgeonholed that his recovery's been meme'd to death and it feels sad. It doesn't help that like Ganondorf Mac's a case of "Good in more casual settings, but horribly exploited by people who really know his weaknesses". There are still pro Mac players, but at this point, it might be worth it to refocus Mac around something other than just ground-based power (he could still be mostly ground-based).

: Another noble experiment here. She's got a couple interesting traits, but I kinda think her moveset's a bit unfocused and she suffers from that. Lloid Trap is interesting, and so is the Fishing Rod, but she doesn't have that much that complements them. Villager's seen as stronger partly because the moves complement each other better (Down Smash -> F Smash for example)... but Villager also struggles in Ultimate's metagame. Buffing her Fishing Rod's speed and giving her infamously short Smashes a bit more range could help that.

: Gale Strike (the Tornado) and the Chakram are just a little too good IMHO, leading the Swordfighter to become Sword-Zoner in certain cases. Especially if the Swordfighter has the Reflector with him. On the other hand, there's not that much incentive for the Swordfighter to actually go into CQC range, with the Swordfighter being relatively shortranged and lacking speed. EDIT: This is where I argue Swordfighter diverges from the three Links - the Link have generally good enough tools and reasons to go CQC too. As such I'd buff the Swordfighter's speed a little and a slight buff to the tilts to compensate for the slight speed nerfs I'd dish out to Tornado and Chakram.
(There are lots of other characters I'd rework, like Sonic. But I've complained a lot about him already.)
*And yes, competitive people can and has lost games and sets to competitive Ganons and Macs. Case in point:
Not sure what they would need to do to ensure a smooth connection for Smash. Not sure Rollback would be it or not. Or if they'd need to figure out something specific to just that mode. Though I've been missing Smash Run for so long. I can't believe that wasn't on the Wii U version. Such a fun mode to be designated only to the 3DS version T_T
I could see that mode work, but it shouldn't be the sole mode in the game. So:
Step 1: Actually include an Ethernet port with the next Nintendo console. Like WTF.
Step 2: If not Rollback, at least upgrade the online infrastructure Nintendo. Especially now when we're paying $20 a year for it. They've been neglecting it for a bit too long.
Step 3: Include a slight damage modifier in the Smash Fortnite Run mode, so people don't risk taking 80 % instantly from like 5 people ganking one person.
It'd be really difficult to design, don't get me wrong, but I don't think it's impossible.