We already have an overhaul for Solstheim in the works,

And Tamriel Rebuilt will soon cover the area surrounding Mournhold,

So do you think a Mournhold overhaul which places it in the game world (not it own cell) will ever be developed. I can't imagine it being much different than the concept for Oblivion's "Open Cities" mods.

Unlikely, considering that in TR, eventually, the city of Almalexia will be finished, the centerpiece of which is Mournhold. TR's intent is to leave vanilla Mournhold alone, and simply build around it. Everybody uses TR, so if a modder were to overhaul Mounrhold, they'd pretty much be wasting their time.