Great points Brent. I think Mattel/DC did have the advantage of seeing what worked and didn't work with the Legends line (both under Toybiz and Hasbro), and like you said, their wave mixes have been pretty well done overall. You didn't see even Toybiz "spreading out" the big names quite as much as Mattel has been able to do. In the initial waves of Legends, you could get Iron Man and Cap in the same wave - today they would be in seperate waves as the "big draw", at least the way Mattel runs the DCUC line.
I think you make an excellent point about the continuation of Legends as well. Aside from a little bit of online complaining, I don't think there would be a problem with re-releasing (or even re-doing) the big guns from Legends (whether its for the kids or collectors, or both) and sprinkling them into "DCUC styled" waves with minor/supporting characters (like the mixes you mentioned). Heck, some figures (the Black Spidey from the Target exclusive wave) was an improvement over the previous Toybiz version I thought - so we might even get some better figures out of the deal. The quality of Hasbro's Legends has been iffy though, but I thought it was getting better (although sometimes tougher to find) with some of the later waves. I especially like the aforementioned Black Spidey, their Dr. Doom, the WM exclusive Torch, She Hulk, and the movie Iron Man off the top of my head.
One thing I have noticed too, even when comparing to ToyBiz's stuff, is that the DCUC figures seem much sturdier to me. They may not have quite the articulation that we saw with Legends, but I sure don't have as many troubles with loose joints and/or falling down with these guys. Not only do they look very nice on a shelf, I suspect they would be great toys for kids too. Anyways, like I've said before, I really like the 3 3/4" scale and it would have been great had they started this way in the first place - but it is getting tougher for me to keep buying them when I already have 6" versions on the shelf. I'll keep up with the Spidey stuff, the Secret Wars packs, and possibly a few others - but I don't think I can keep up with the MU in its entirety anymore (I do want that "Amazing Friends" 3 pack though, loved that show as a kid as cheezy as it might have been).