So Oasis is not going to be a sand town? What a pity! I also thought about balance issue already, but hoped it's gonna be the first desert town anyway. At the moment, in SoD and HotA, desert is used as "neutral" terrain because no town is native to sand and because sand also has movement penalty. I guess that's also the idea behind the new dunes from HotA (because no one can see the quicksands in the battles).
But I think it would still be possible to have Oasis as a sand town. Because there are more towns than colors / players, not all towns can be used on a map. So if on a map e.g. Stronghold is disabled (by the mapmaker), Rough terrain might serve as a "neutral" terrain for THAT map. On another map, Oasis might be disabled (for sand) or Tower (for snow) or Fortress and Cove (for swamp). This mapmaking strategy is already used for some campaign or scenario maps I think. With such a procedure, it should be no problem to have a sand town included in HotA. Those existing maps, which use sand as neutral terrain, but have only fixed factions (so no Oasis) would not be affected anyway. And sand fits really best for Oasis, no doubt!
If Oasis terrain despite all that will not be sand, it's going to be Rough, I guess?
PS: Lion assassin looks nice