I'd say that if Olimar needs anything as far as balance is concerned, his pikmin needs to be invulnerable during usmash's start-up, so that Olimar can't be stuffed out of usmash by any move in the game when he's trying to use it as an anti-pressure option. It's also prudent to nerf whistle armor by making the window smaller, and making the full move last longer, so there's more reward for timing it and less reward for spamming it. Pluck absolutely should not be slower. Pikmin are Olimar, and nerfing the ability to have Pikmin at all times takes away Olimar's ability to be a character.
As far as more aesthetic changes, I think there should be more incentive and tools for line management. Line management is a big part of high-level Olimar play, but really the rewards are usually not very huge, and the tools are incredibly limited.
Now, this discussion necessitates a bit of a detour into the randomness of Pikmin summons. So there's a bit of a double edged sword here. The randomness of Olimar's Pikmin makes their usefulness dependent on a roll of the dice (or more accurately a cycle of the RNG). If you randomly pluck six whites against Fox, then that sucks for you. In that light, the randomness of the Pikmin can be a huge bane for Olimar if he gets unlucky. The inconsistent effectiveness of Pikmin plucks is what necessitates line management at all. Line management allows the player to turn his line from a useless bunch of Pikmin into a viable gameplan. So that presents a dilemma for people who want to get rid of randomness: If you get rid of it, the most interesting and involved aspect of Olimar is no longer necessary, and Olimar's game becomes even more one-dimensional.
So we can't just get rid of randomness. We have to mitigate it. Brawl did this in two ways. First off, it allowed the player to control which Pikmin were in line at what time by using Whistle, and by throwing away Pikmin he didn't need. Pikmin attacks could also be used to subtly alter the line order. The problem is, this doesn't really provide a full repertoire of line management options. Attacking to change line order is a huge time commitment in a fast-paced fighting game, and killing Pikmin is a slow, painstaking process that leaves Olimar weakened for much longer than necessary. Because Olimar lacked effective methods of certain line-management options, randomness could still hinder Olimar, especially if he plucked a Pikmin that was more a liability in a given match-up than an asset.
So Brawl also employed a second way to make randomness less impactful. It homogenized Pikmin. While Pikmin do have different properties in Brawl, only whites and purples are particularly specialized. Reds, blues and yellows are by and large interchangeable. If you have a yellow next in line but would like a red for the better attack damage, that's okay, because the difference isn't much more than 4% on most attacks (uair being a notable exception). Stuff like that makes it easier to be less worried about the Pikmin that's next in line.
Rather than go with a homogenized Pikmin line and fewer options for line management, I'd rather go with improving Olimar's options for line management, and making it more necessary and rewarding by giving each Pikmin much more specialized roles.
I'll start off with line management:
UpB: Pikmin Footstool - Olimar footstools off of one of his Pikmin, killing it in the process. This isn't really my idea. It's been floating around a while, and I realize that it's pretty much impossible to implement by having Olimar literally jump off one of his Pikmin, but I do think it should be possible to make it look like that's what he's doing. What I like about this is that it can be done such that Olimar can react very quickly after he's performed the jump, allowing him to waveland after a grounded UpB, making farming very quick and safe. Naturally, it also doubles as a recovery move that can be acted out of, but has a cost.
NeutralB: Pluck - The move is no longer useless in the air or with a full line. In either of these situations, rather than pluck a new Pikmin, Olimar rotates the line by one. This is distinct from Whistle because Whistle not only recalls thrown Pikmin, but also always puts them in the same order, with a pre-determined color at the front. Futhermore, extend the utility of smash pluck canceling by allowing the ending lag of pluck to be canceled by any move that uses Pikmin--any of his smashes, grabs, non-neutral aerials, or specials, basically. This makes it much easier to incorporate line management into Olimar's offense, so that he doesn't have to let off the pressure in order to get the Pikmin that he wants.
The rest of Olimar's specials are fine as they are. So what about specializing the Pikmin:
Red Pikmin are Olimar's Attack Damage Pikmin. They do the most damage on normal attacks, but aren't as effective with throw or latch damage, and they have poor health. They also have sub-par knockback growth, beating only Whites in that category. This does make them more effective in combos, however, especially at percents where other Pikmin would be knocking opponents too far. They also have less range and active frames than most Pikmin. Also, usmash and uair are meteors on grounded opponents (dair is always a meteor, so there's also that).
Blue Pikmin are best when throwing opponents. They have mediocre health, and on attacks an latches they have mediocre or even poor knockback, damage and range, but they have the longest grab-range, the second most damaging pummel, and the best knockback and damage on all throws.
Yellow Pikmin are more used for zoning and space control. They have the largest and longest-active hitboxes of all the Pikmin, and the longest range on fsmash. Toss retains the higher arc from Brawl. Otherwise, they have mediocre damage and above average health, and the knockback is best for its low angles.
White Pikmin continue to be the latch DPS Pikmin. They have the worst health, and they have the worst damage and knockback on attacks and throws, and their range is mediocre, but their pummel and latch damage is unparalleled. They also retain their long Toss range.
Purples also continue to be the tanks. They have great health and phenomenal knockback growth. They also have pretty good attack damage, second only to reds. However, they have terrible range, and they aren't great for throws. They continue to just hit opponents when thrown rather than latching.
With these changes, randomness is further mitigated by having many more options to counteract its effects. This allows the properties of the Pikmin to be more specialized, which in turn makes it more necessary for Olimar mains to become more proficient in line management.
