MoonSong
Rules-lawyering drama queen but not a munchkin
I think I've mentioned it a few times already, but I think my ideal multiclassing system would be a feat-based system as a prerequisite for taking levels in a new class. In order to multiclass, you need to first take a feat that would give you some of the abilities of the new class, and you might also need to meet other prerequisites, e.g. training in a Knowledge skill for a wizard, or proficiency with heavy armor for a fighter. This also has the added feature of making it easier for a cleric of the Warbringer to multiclass into fighter, while a wizard would have to pay a steeper cost.
That would severely limit multiclassing and bring us back to "plan your build", I know it sounds good on paper, but in many cases that means a heavy investment in feats before being able to multiclass and would somehow stereotype characters. if after a life changing event my third level rogue decides she wants to reform and study to be a cleric she has to earn four more rogue levels before being able to take the first level in cleric (two whole levels before being able to take the special multiclass feat and two more to take skill trainning into knowledge: religion), that means she not only has to keep going on the same path before being allowed by game mechanics to branch out (cannot take cleric levels or any other levels in the meantime so has to remain a rogue) will be a less viable cleric over time, too draconian for my tastes