Sort of "webkit for email", that would be interesting... The problem is finding a decent monetization model.

Btw, we kinda had something like that: Lotus Notes and MS Exchange, originally, were abstract document stores. In theory, anyone could have built custom views that behaved as you wished, while the platform took care of email fetching and synchronizing. Unfortunately, both Lotus (later IBM) and Microsoft saw those platforms as lock-in instruments in a push for vertical dominance, so they had no incentive but to push their own clients.