One of the things I noticed in the new comic
The Nightborne: Twilight of Suramar was that the Legion felt they could pull down the shield, but it would cost the nightwell to do so, but they need the nightwell for something ... and so give the nightborne the ultimatum.
All this is happening just before we arrive on the broken shore. Gul'dan hasn't had a chance to do much we know that, Thalyssra has just been expelled when we meet her, a few days only.
Why are the well's so important, this one is ofc the same substance of the Well of Eternity, whether they magical pooled some vials here in Suramar to create the font in the palace, we know the night well is formed when the Eye of Aman'thul is used 10k years ago, to converge the leyline nexus into the magical font at the palace - we know the leylines are the magical energy of the world, this is like Azeroth's arteries or veins and the well of Eternity was a wound, that resulted in giving life to Azeroth specific and only lifeforms as well as converting other titan life forms or shall I say adapting them to an Azerothian state.
Is there more significance to the Well's of Power? Is Gul'dan trying to affect the titan itself? I can't believe he is that aware of what's really going on, it seems that only Sargeras and his top lieutenants are believers in the cause, most of the others just want more power and / or more chaos and destruction - afterall that is the nature of the beings of the twisting nether completely given in to their desires wantint in their base passions to consume everything.
Not to over think it, but I think I conclude that Gul'dan wants the power himself, but it would seem that he has need for it to achieve his ends.
HE completely bluffs the nightborne, Thalyssra doesn't fall for it, a few others don't, but Elisande does, and when she does, the advisors who agree with them turn on those who do not. And so the rebellion is started.
What Gul'dan wants though, who knows.