I've found this problem - the issue seems to be that when you clear out all the creatures, the land doesn't "heal" or rather whatever script they have that removes the influnce bubble from the Imperia doesn't execute properly, which is odd, as it work for me on the same map with a swampy scarred land. Also, is it possible to have the special areas border be some other colour? I kept confusing it with Tarth green, though perhaps it is just me.

The Revitalization Spell (used to restore land) is missing along with Raise/Lower land are gone in the  .77 patch!!!

I've got raise/lower in .77.  No revitalization, though...

Ran into this problem as well.  But move a pioneer near there anyways.  When you mouse over the settle button, look at the map, there should be 1 (or 2) locations in the imperium that light up with the food/material display.  you CAN go there and settle even though it is in "enemy" land.  That will complete the quest

I have 0.77 and have raise land at least, used it last night to get past god damn caravans that blocked me in on a narrow road, I couldn't move.  

I had a wildland with a city in it, captured, it but it never produced influence and it did nothing special other than having a darkling defender that I could tell.  Also, the one with the big worm never went away when I killed it, big green question mark on the mini-map forever.

I don't know if I have that one, as I didn't look.  I can only verify that I absolutely had raise land and it's working fine.

I've had great luck revitalizing the wilds.  You seem to just have to clear out all the monster dens, then the hostile territory goes away and you can resettle.

I have had it happen twice with the big world quest zones, clearing all mobs including the big bad boss does not allow outposts to be built as the zone still belongs to the monsters - because I reloaded. Once with the big ugly green swamp monster, and once with the ice monster (sorry, cannot recall their names ). It is like Rishkith says - when I first move into the zone, a quest will trigger and I have it displayed on the left of the screen below the city list. If I save and reload in the next play session, the quest is no longer there. Perhaps the save file has not saved it correctly, thus when reloading the quest is non-existent and will not trigger completion and the zone forever remains with monsters. Can the developer please check and verify?

There is only one tile which is settleable.  If there is any city too close to it, it prevents it from being settleable.  So you need to check yours or other players' cities to see if any are too close to the wildlands.