In all likelihood, the 5.0-liter V10 in today's M5 will be replaced by a high output version of the new 4.4-liter V8 like the one in the X6 M. However, this 555 hp monster will probably be backed up by the mild hybrid system that will be installed in the 755iH late this year. Unlike other applications, BMW may add a power on demand button of some sort that uses the available energy in the battery to provide instant boost. Rumor has it, BMW is also considering adding two cylinders to make the V8 a V10 and then using cylinder deactivation to improve mileage. This seems a much less likely scenario since the increased torque of the V8 compared to the current V10 combined with the hybrid power should provide plenty of extra performance at a lower weight than adding more cylinders.
[Source: German Car Zone via BMWBlog]