It would offer more flexibility, sure, but I don't think it would make the game better. 5e...isn't really designed with multiclassing in mind. It's an optional rule, and there are lots of exploits that result from multiclassing---mainly due to classes being so frontloaded = huge benefits from taking even a single level. E.g., sorcadin, sorlock, caster + 2 levels in Fighter, starting with Fighter to get Heavy Armor Proficiency on a caster. Ability score requirements to multiclass out of and into a class are the only thing holding these back.
Additionally, while some combos would be OP, there would still be many other suboptimal, non-synergistic multiclasses that people might take if there weren't at least some restrictions.
