I think the best idea would really to use a composite of pretty much everything suggested thus far.
Use story awards with lowered values to slow progression, and then perhaps break the levels up to give them out piece meal. Using NPC classes would be interesting, although I think you'll find a lot of warriors and experts. Then again, I could be very wrong.
The biggest worry you should have is your players. D&D is designed to continually give you stuff to make you feel good. Look at the new revision, they tried to make sure classes got something every level because players _really_ like that. Giving out magic items isn't the best idea because that is basically the same as giving out levels in terms of giving out power.
I think the best way to do this would be to make a really intriguiing story where characters earn allies, friendships, RP benefits, titles and so on. This way people get a lot of rewards and benefits other than character level.