0-60 times and HP ratings are not what cars are about
the e60 M5 engine was astonishing because of its high revving linear power curve, not just because of its output.
if the M5 needs an increase in performance, then BMW should pursue that by maintaining the same philosophy and approach for which people love M cars in the first place.
Also, don't forget - the current M NA engines don't have direct injection yet.
here is my ideal for a new M5:
-Direct Injection V10 with ~550hp, 350lb-ft tq. redline @ ~9000rpm (wishful)
much improved fuel economy, and reduced weight
-Adaptive Drive System (anti body roll) and Dynamic Performance Control (torque vector) implemented without making the car feel too artificial and computerized.
-Overall reduced weight. (both from lighter base 5-series, and from liberal use of carbon and aluminum)
-Much increased level of track tuning for hardcore performance
-Much higher level of aero work. This will help in many way. First, it would further visually distinguish the M5 from the normal 5 (good for today's marketing playing field?). Reduced drag and lift would improve stability and reduce wind noise. It could also allow for an even higher top speed (more marketing clout?).
BMW could even take it up to another level by introducing ACTIVE AERO. possibly a retracting front spoiler or an extending rear spoiler (similar to the SL65BS). This would not only help performance, but is a pretty cool talking point.
anyways, if this direction was followed by BMW, I think the M5 would remain as a genuine super-sports sedan, unique from the RS6 and E63AMG.