I hope something that isn't player vs player based. Something entirely cooperative. Just you and your teammates working towards a common goal. (Would be okay with something solo too, but seems unlikely they'd add an entirely solo gamemode at this point.)
Tycoon-style gamemode would be interesting to me, because besides Minecraft I mainly play Bethesda and simulation/tycoon titles. However, I can't really picture how it would be done in Minecraft (don't want a click and wait "tycoon").