Lanefan
Victoria Rules
The split xp progression is exactly what I'd want, as it then allows players to determine how much emphasis is to be put on each class by varying the % of xp that goes to each one. I'd also like to do away with additive levels and have the two classes function independently as per attack bonus, saving throws, etc., such that a Cleric-5/Fighter-3 fights like a 3rd and casts like a 5th and saves as whichever of those classes gives the better save in that situation and is in no way an 8th at anything.I'd prefer hybrid multiclassing, because it would feel more like 2e multiclassing, though I definitely don't want to go back to the split XP progression.
There's a big difference between a character who puts 75% of xp into Fighter and 25% into Rogue and a character who puts 75% into Rogue and just 25% into Fighter. The first is mostly a warrior who wants to do some sneaking and trapfinding on the side, the other is mostly a sneak who wants to augment her fighting skills. The expectation is that a character is played roughly in line with its xp division. The xp division can only be changed between adventures and is otherwise locked in.
In no case, however, should a multiclass character ever be more powerful than a single-class character of the same (highest) level.
And yeah, as none of this is even remotely backwards-compatible I'm not holding my breath waiting for it to happen.

One way to put a hard stop to a lot of this is to allow any character to only ever have two classes, tops.I'm not a fan of those ridiculous multi-classing for various level dips builds of characters.
@roger semerad - the biggest problem with feat-based multiclassing is that it even further bakes feats (which are/were in theory just an option) into the game; hardly a solution for those who don't want them.