We all know how Black Clouds and Silver Linings had, in its limited edition, not one but TWO bonus discs, one was the instrumental version of the album and one was six brand new covers. Let's say on the next album that will be the followup to The Astonishing we will get a bonus disc... what would you want on that?
Since this is all highly hypothetical anything goes for the replies of course. As for me, I'd love a Rudess / LaBrie combo. Just the other day I casually refreshed FB and Rudess was live playing an improv piece on his piano. I'd love for him to sit down with James and conjure up something only piano and voice. Have the bonus disc being a Rudess / LaBrie EP, with 2-3 original songs, a couple of covers and a couple of older DT songs reworked and rearranged just for piano and voice, kinda like the alternate version of Wither.
As a second choice, I remember the acoustic shows of the FII era, they never did something like that before or after again, it was unusual and interesting to have those kind of performances. I would love to hear some of that again, just like in my JR / JLB only scenario they could apply the "New songs + DT songs rearranged + covers" scenario to have 6-7 songs for a bonus EP.