Realistically, there is only 5 or so names on the current roster that have the star power and stand any chance of winning the rumble and headlining mania:
John Cena
Roman Reigns
Daniel Bryan
Seth Rollins
The Undertaker
Taker doesn't need it and Rollins is out. Either Reigns or Cena will be champion going into Mania which leaves only a couple of realistic options. Outside of that you are looking at part timers such as HHH, Lesnar, Rock etc.
All your 'second tier' guys that should have a realistic chance of winning - Ambrose, Cesaro, Owens, Wyatt, Ziggler etc don't stand a chance because the prize is competing at Mania for the main belt and 90% of the time - headlining the event.
Since the second world title was scrapped, Royal Rumble winners have been very very predictable. I think they really need to scrap the prize of a 'Mania match'. Winning the rumble should be a good enough push in itself if done right. Even changing the prize to 'A guaranteed PPV main event for the title' would open up the rumble to a whole host of different options.