Yeah, I was going to say that 4th edition did multiclassing pretty well. I actually forgot paragon multiclassing was a thing, but the later hybrid system was especially fun and seemed extremely balanced. 4e multiclassing worked a little bit like the 5e bard's magical secrets -- instead of starting at the beginning of a class, just take a limited number of higher level abilities instead of one from your own class. Nixing a la carte level based multiclassing and making multiclassing feat based would probably please no one, but it would potentially be more balanced than the current system. There's a reason multiclassing was technically an optional rule -- the designers never claimed that it would produce even results.4e.
4e multiclassing is very consistent. You should always take one multiclass feat unless you literally can't spare one. You should (almost) never paragon multiclass, because it sucks. Hybrids have a few basic rules (have at least one stat shared between your classes, pick two classes of different roles in most cases, keep in mind what benefits you can get via feats, e.g. Hybrid Paladin can take a single feat to get Plate prof.)
Taking 1 MC feat is always good. Two is a sometimes food. Three+ and PMC, inadvisable.
Plus, feat-based MC isn't too far off from 2e's Dual-Classing, just skipping the "re-level through 8 levels of the new class" part, and Hybrids are straight-up 2e dual-classing with better balance.