If you took Splatoon 3 as-is and allowed custom kits, it would likely be broken. Burst Bomb combos would be everywhere.
But I think if Splatoon 4 was designed from the ground up with custom kits in mind, it could be a lot better than what we have now. They'd have to make some changes to accomodate a new system, but it'd be worth it in the end.
Really, I think anything could be better than what we have now. S3 has done such a terrible job with kits that I do not want to see this same system return in S4. Replace it with something else, anything else.