sd_doomsday_event?
That makes sense, and would continue the Valve tradition of using "crappy" maps for events.
I don't think they are ever going to do a straight A/D type of map (regular Payload included), as those tend not to be symmetrical, and I don't think that they'll do a CTF map, because they're terrible stalemate fests, and if anything, Valve does everything that they can to make sure that the maps won't be a continuous stalemate (see the improvements made to Hightower for the event to get an idea of what I'm talking about).
So that basically leaves Payload races, KOTH, and the much hyped, and then immediately forgotten Doomsday map, with Mann Mannor being a noteworthy exception, as that is an A/D map.
Wouldn't put a Teufort event or Dustbowl event map past them, as both of those are classic TF maps, going all the way back to Team Fortress (the Quake mod). And now with the TFC mercs being introduced into the TF2 universe proper, wouldn't put it past Valve to make a "throwback" event map one of these days (but probably not for SF 2014).