They do something like that every tour pretty much. There's always a surprise in there. Maiden England had Afraid to Shoot Strangers, which hadn't been played since the 90s, SBIT had Moonchild, TFF had the 00's focused setlist, AMOLAD had the whole album + Children of the Damned on the 2nd leg. Early Days had a ton of unexpected tunes.
Flight of Icarus makes sense, if they can agree on a tempo.
I doubt it'll be an epic length song, even though something like TTAL would be pretty cool. Maybe they'll try Infinite Dreams again? I could see them going for something off SIT, like Sea of Madness maybe. If they're gonna reference it on new albums they might as well do it live.
Either way, I'm gonna guess it's going to be an 80s song that didn't get played on any of the history tours.