*cracks knuckles*
- Realistic environments. No more kiddy, blocky or colourful worlds. They are getting old. A post apocalyptic setting, for instance, would be nice for a change.
- A new and deeper story. They need to stop re-using the same old "Oh no! Bowser has kidnapped the princess!"-story. Been there, done that. It would be much more awesome if something like aliens or terrorists started threatening the earth, with Mario being humanity's last hope-
- The same old gameplay mechanics needs to be completely revamped. Simply running and jumping sideways does not offer enough variation. Preferably, I would go for a 3D world, seen from a first person perspective. That way you can see what Mario sees, feel what Mario feels and truly experience what Mario is experiencing. I mean, why hold back the realism when current hardware have made this much progress? We need to stop living in the 80's.
- New-, and better power-ups. We have already seen the ancient fireflower, and it's about time they mix things up a little. Adding guns and rockets sounds like the ideal choice here, as it fits perfectly with the setting and FPS view.
- New characters: Let's take a look at Mario for instance. He will never get rid of his kiddy image. As such it would make no sense to keep him in his current form when the series is in this desperate position to change for the better- more mature audience. He could keep his name to help push sales, but the character would need a new voice, an actual background story and a new job. Preferably as former member of a mafia gang, to give him a reason to be experienced with weapons.
(I have a feeling though that even by that point people would still call it an unimaginative remake of Super Mario Bros.)
