Well, all the albums after Scenes were "occupied" by Portnoy's AA Suite, DT11 would have been the first chance to make a concept since probably they started to get "Hey, why not" ideas already around then, but then of course the split happened and the new musical direction with Mangini had to be estabilished...
Had not Portnoy left, I think DT11 or DT12 could have been concept albums, without waiting for DT13. Now for sure the next one won't be a concept and most likely the next after that, but from DT16 onwards I don't see no strict rule that forbids them to do another concept if they want to...