This proposal presents a "middle ground" approach to improving and refactoring auth.User, based around a new concept of "profiles". These profiles provide the main customization hook for the user model, but the user model itself stays concrete and cannot be replaced.
I call it a middle ground because it doesn't go as far as refactoring the whole auth app -- a laudable goal, but one that I believe will ultimately take far too long -- but goes a bit further than just fixing the most egregious errors (username length, for example).
This proposal includes a fair number of design decisions -- you're reading the fifth or sixth draft. To keep things clear, the options have been pruned out and on the one I think is the "winner" is still there. But see the FAQ at the end for some discussion and justification of various choices.