A set order of Pikmin is a strictly good change compared to the RNG he has now. Olimar's kit is one of the worst designed in the game with how convoluted, random, polarizing and redundant his key tools are.
There's no reason for him to have a random order of Pikmin at this point because all it does is cause players to be at a huge disadvantage or advantage at any point during the match by giving Olimar players such horrible tools for setting up their Pikmin lineup. Having to farm indefinitely until you have a favorable lineup for the specific MU you're in is stupid when you only have fleeting chances to pluck Pikmin that could also potentially make your lineup worse; this disparity is one of the most offensive things in this game imo. Sometimes it works in your favor, but other times it's ******** how ridiculous (in both senses of the word) of a lineup you can get in such an arbitrary fashion. A set order of Pikmin would relieve this issue by creating a sense of consistency within Olimar's lineup and frankly his kit as a whole. Olimar players shouldn't be penalized or rewarded in such a random fashion when the viability of their character hinges on this one aspect.
If a set order did end up making it into the game, I think that a few other changes to Olimar's kit would be necessary to give his players the most cohesive experience possible. First, I think his Pikmin pluck as well as his whistle would need to be sped up in order to give his players more of a chance to alter their lineups since they would be required to do so on a much more consistent basis. Second, his whistle should be relegated to shifting his order of Pikmin by one, so a lineup consisting of Red, Purple, White and Blue would always shift to Purple, White, Blue and Red after whistling. Personally, I find this way to be much simpler, obvious, and more beneficial than the way his
whistle currently works. Having a whistle that shifts Pikmin in the same way as doing a move does is much easier to understand and allows you to control and maintain the variety of your lineup which leads to you having more options at the end of the day. Third, his antennae should indicate the next Pikmin he is about to pluck. There is no reason for it to indicate the next Pkimin you're about to use because you can already clearly see that. The way it works currently is simply redundant and ends up helping Olimar's opponents more so than it helps Olimar's actual players. The final change I would give Olimar would be to increase his lineup capacity by 1. I'm not exactly confident that this change is necessary, but I do think it would be beneficial to Olimar by allowing him to have a greater variety in his lineup which, again, would lead to him having more options.