There is a similar feature request to this at:
viewtopic.php?f=3&t=525

But I thought I'd start this thread because there's a different version of it that would be helpful and that's just being able to preview (not edit) animations as they mix together. Let's say you had a jump start, jump loop, and jump land animations ... in the editor it would be great to be able to preview this sequence with mixing settings between each of them.

A possible UI idea would like a multi-layer video editor like Final Cut Pro, that is a timeline where the dopesheet is with multiple horizontal tracks. You could drag and drop an animation onto a time in one of the tracks which might create an animation clip, you can set clips to looping and drag to resize their loop length. Dropping other animations on other tracks that overlap in time with placed animation clips would be a mix during the overlap in time.