Being able to move, delete or edit mountains, rivers, etc.
In my opinion the game is very good as is right now, sure it can have some more features, but what I think lacks the most, is variety, you can go on 20 different servers but 90% on all of them will look the same. The biggest problem ATT has right now, is that its so repetetive even through other servers.
I think once we get to the point where we can not only do add terraform (through command console), but edit and erase terraform aswell, then we will see this game skyrocket into new heights it hasnt seen before. I like to think of it as minecraft, the reason minecraft got so insanely popular is because every single server can be completely different just by the game itself, not including mods.
I really do belive, once general terraforming can be implimented even in the slightest, this is when ATT will exponentially grow.
My problem with house building, is that it is looked at like it should be the main focus, but I think Housing should be a side effect not a main focus. With terraforming in the focus, house building will come by itself, even with a little bit of terraforming. The creativity of ppl can work with that and create houses of of little things terraforming can do.
Oh and this will obviously take a long time, this will be the most time consuming feature there is, period. But it will be worth it, this, I can say with certainty.