Drop the Dinan tune on the N-54 mill and the new Z4 will be a screamer. I have Dinan in E93 and it really flies above 3500 rpm.
Agree unlikely to be dedicated coupe or (in US) ragtop either. But folding roof is super slick and weight penalty for 2-panel version in E89 may not be horrible - TBD.
M version? Pretty easy to tune up the N-54 (Dinan hits about 385 HP and around 420 lb-ft at flywheel) and put some other goodies on it to make it a legit M version. Will depend most on whether BMW hits the mark with the basic chassis: if the basic N89 is a dog, any M version would be mostly marketing; if it's a solid sports car, M is easy.
Remaining questions (my guesses in parens):
- Weight? (3250-3500#)
- Cost? ($53-56K loaded)
- Pricing available? (March)
- Active Steering? (probably as option)
- Electronic 'locking' rear end (probably)
- non-M-DCT? (probably as option)
- Steptronic? (yes as option if no DCT; no if DCT)
- Latest iDrive/Nav setup? (probably as option)
- Adaptive headlights? (maybe not; base if yes)
- Run-flats? (probably)
- M seats (probably as option)
- HUD? (no)
YMMV...