They'll never do what I would like, especially with what we've seen thus far. I'd like 1E/2E style multiclassing, but not fixed at 1st level and without the exponential XP tables. Instead, I'd do it as any character can pick up a level in any class they want, any time they want, which progresses independently. But the XP charts are smooth enough that going very far in more than about two classes really starts to eat into your progression. So I guess a 1E/2E/3E hybrid, with a few twists. Backgrounds and specialties already handle the 4E-style dabbling well.
However, the XP chart would look something like this: 0 level - 5000, 1st level - +1000, 2nd level - +2000 more, etc. You get the first class at 1st level for free. After that, you pay a premium to get one started, but then advance normally from there. That structure only works if you keep the increase each level relatively slight. But then, I've never been a fan of 1,000,000 XP at some higher point. (Heck, I'd drop a zero off of all of them. It's just not all the fiddly tweaks for monsters to start at 1,000.) That also doesn't work well if they go into something like 36 character levels, but I doubt the progression will go that high.