Progression realm Vanila - Mist of Pandaria
I already played in Lordaeron at launch 2016 where it was an improvised progression realm with temporar caps at 60, 70 and so on. Now you are working on Onyxia which will be a better suited progression but still it's a bit boring for me because I already did it. But then I thinl if you expand the classic formula to include everything to MoP somehow I think you will attract a lot of new players. You already have a head start because you was running all these expansions and have the code, now you only need to find the easiest way to implement the progression element and polish all the major remaining bugs which is a lot of work but it might bring a lot of new players.