Although I'm not the biggest fan of it, Empire of the Clouds is a blatant candidate. It's got lots of fans and is a truly unusual and - dare I say - standout tune. When it comes to earlier stuff, I think Only The Good Die Young and The Loneliness Of The Long Distance Runner (although the latter has already been played once) are quite dear to lots of Maiden fans (me included). Personally, besides the aforementioned OTGDY and TLOTLDR, there are five songs I'd love to see debuting live: Montségur, The Prophecy, Prodigal Son, The Thin Line Between Love And Hate and Back In The Village - again I think the last one has also been played live once. *
Then when it comes to songs played for just one or two tours, I'd have to go with Still Life, Infinite Dreams, To Tame A Land and I'd simply love to listen Bruce singing Fortunes Of War.
Quite frankly I seriously doubt they'll ever play any of these tracks in the future. If I had to guess it the one that has the best chances would be Empire Of The Clouds IF they decide to give some kind of concert with an orchestra.
* Other songs like Invaders, The Duelists, Flash Of The Blade, Deja Vu, Fates warning, Run Silent Run Deep, Face In The Sand or The Alchemist would also be cool.