I'm not a fan of the rear-end either, but I think it's important to point out that this
is not a BMW design, it's a Zagato one.
Overall I think it looks great and very Zagato. All the hints are there: Upswept window line, double-bubble roof, rear window in a U shape etc. That's what you'd expect
Zagato to do. Here, have a look at
this.
The point I'm trying to make is this is a kind of a coachbuilt design (kind of like how you could get the same Ferrari dressed by Pininfarina or Bertone). Zagato took the BMW and imprinted their own design philosophy on it, just like they've been doing for years and years.
Just accept this for what it is is what I'm saying. It's a BMW Z4 by Zagato. It's not an official BMW lineup, it's not a new BMW lineup, it's nothing but a styling redesign by an Italian design house.