One of the things I'm considering for my race design system is what to do about multi-classing. Elves in Men & Magic had a multi-class vibe, although there has been some debate as to whether they were supposed to be simultaneous Fighter/Magic-Users or start with one, then switch to the other. In Greyhawk, the answer is clearly the former, which resolves some uncertainties, but creates another problem: we now have two systems (multi-classing and switching classes) reflecting an artificial divide between non-humans and humans. Also, multi-class races lose a little something, flavorwise.
What I've been using is an expansion of the standard class-switching rules. The rules for switching to a new class are basically the same for everyone, human or not. What makes elves elves is the fact that they can switch to or from the Magic-User class repeatedly, and they don't need to worry about having a minimum prime score. (Delving Deeper also goes with this interpretation, so it's not just me.)
So what I'm thinking of is going with that interpretation for all races. A race can have a favored class, in which case they can switch between that class and another class every adventure. They only gain experience in one class at a time, and they only get the hit dice of one class, but they don't lose the abilities of their other class.
This does change some things, though. Double-class characters are simple, but triple-class characters are no longer an automatic option. An elf can switch back and forth between M-U and Fighter, or M-U and Thief, but not between Fighter and Thief. So, under the rules I'm thinking of going with, triple-class elves are impossible by default, although it's possible to switch from Thief to Fighter once with a Strength 16+, or Fighter to Thief once with a Dexterity 16+, and continue to switch at will between the second class and the Magic-User class, but not back to the first class. Or, with both Strength and Dexterity 16+, the elf can switch at will between all three classes. Thus, elven Fighter/Thief and Fighter/Magic-User/Thief are possible, but rarer.
That is a little more complicated, but it does have the benefit of using just one system for switching classes, and it lets us design alternative races using the same classes, but a different preferred class. A race of "spellbrutes" might have access to Fighter, Magic-User, and Thief, but with Fighter as the favored class, which means that the max levels for Fighter and Magic-User are swapped, and Magic-User/Thief or Fighter/Magic-User/Thief becomes the rarer combination.
I'm still considering whether this change is worthwhile.