I don’t think The Undercity is bad, strategically. It is a well built fortress and were it not for Jaina, the Alliance would’ve been repelled all together, as is its intended function of defeating 9 to 1 odds.
While it is ‘in the middle of nowhere’ (they controlled the territory and the blood elves’re up north for immediate relief), a key thing to note is that it is only a problem for supply lines, which the Forsaken do not depend upon as living creatures do. They do not eat, they do not sleep, you cannot drive them out by smoke or pestilence. They are ideally the perfect army for holding a location or laying siege to one, because the only way to reliably take a fortress is to starve out its occupants, for trying to take it by force is exceedingly deadly.
What Lordaeron needed, above all, was First Arcanist Thalyssra and Grand Magister Rommath to have a wizard fight with Jaina.
I don’t think the Forsaken should make any plan that the enemy will deal with them in good will, nor should the Horde. With everything that’s transpired, there is no way nobody is going to be happy with peace, save for maybe those that’re tired of fighting, or have seen enough to realize that to go back to war would be to invite all this unnecessary suffering a second time.
The Forsaken especially cannot make that gamble, for they don’t procreate. There is no coddling or second chances for them. Each death is one unit that is difficult to replace.
Stratholme is cursed to forever burn, I don’t think it’d be ideal for the Forsaken to try and move into it.
My money’s still on Andorhal, if Undercity proper has been blown to smithereens by the kinetic energy of those blight bombs going off.