What I am proposing, is creating simple check-box like with hardcore, when you create character. You would simply tick AH Character on and off. And what to do with old characters? Allow one time only switch for them.
But now, how to motivate people into playing without AH, if they can play with it and buy eq? You have to hang the carrot on a stick little closer to their nose. So what if, for example, all non-AH characters got flat passive magic find bonus, let's say at least 75% so it will make huge impact right of the bat.
And one last thing, I would leave commodity functionality of AH on for all characters, since trading gems have become really pivotal and biggest achievement in terms of AH being good for a game.
I am sure this would not be that difficult to implement in future patches and would really propel the game in new direction and give many people reason to come back.