Don't change the exterior styling too much, use the formula of the 3-Series - make subtle changes that update the look.
Seriously consider what the competition is doing, specifically Porsche. From what I've read, the new Boxster is no joke and can no longer be considered the baby Porsche or a Porsche when you can't afford anything higher than a boxster.
Hopefully BMW can really hit the next Z4. Everything about it screams driving machine - BMW make it one please.