I don't think it's so much that TBOS went backwards as much as there wasn't really much more they could do with the stuff on the past few albums.
Yes, well perhaps they could have a go at thrash or power metal, less the double bass.
Something a bit quicker and heavier like Brighter than a thousand Suns, or Be Quick or Be Dead stuff. Perhaps inspired by Helloween's Keeper of the Seven Keys (song), or Metallica's Master of Puppets (song) or Megadeths (Tornado of Souls), Sepultura's Arise (song)
Or maybe even go symphonic like Nightwish's I want my tears back, or Within Temptation's Angel or Fire and Ice.
Or perhaps they invent some completely new style.
Obviously they could completely alienate their current fan base, but I think they are so talented that they can perfect whatever they try. They'd probably be more influenced by 60's and 70s bands than anything in th elast 20 years.
I guess I'm less interested in Maiden just doing more of the same. Not that I have grown tired of what they have done, but more that I want to see what else they are capable of.