It will probably be between the V6 and the N54 I6, because the other engines have more fuel consumption due to higher displacement. Space fitting issues too. I don't think BMW will go over 3.3 liters due to efficiency reasons. The lowest cost of development will be on the current TT V8 and the modified N54. And the TT V8 does not fit the fuel consumption bill.
And BMW couldn't pass up the track record of the N54. The R&D on engine longevity has already been done by the thousands of tuned N54s which hold on incredibly well at 420+ hp, and show no additional unwanted driveability changes (turbo-lag, etc.) compared to stock. How many prototypes could BMW use to test the engine longevity ? They surely can't match the sheer number of tuned N54s
If BMW replaces the fueling system with a larger capacity one and upgrades the turbos, and maybe increase the redline to 7500RPMs, this could be a winner.
Also, in my country the taxes go incredibly high on engines over 3 liters of displacement. This is probably the case in other countries too. Maybe BMW should also consider this aspect of the M3 cost of ownership.