Basically, Sylvanas has been terrified of dying again ever since the Val’kyr brought her back, because, apparently, she didn’t like where she ended up.
With the information we now have, about how The Maw super sucks and is where all the worst people go, I think it’s safe to assume that The Maw is where she ended up.
The Maw is also apparently where this super evil Death God is imprisoned.
So, what I think her plan might be is…she goes through all this in order to free the Death God and then gets us to kill it. You know, like we do with every other imprisoned god that gets released when we’re around.
Either way, either from freeing the death god, or by killing him, his prison, The Maw, is destroyed as well. And if The Maw is gone, then she can’t get sent there next time she dies. And neither can anyone else.
Even if we do free their souls from The Maw, that just means they’ll get to go to their proper afterlife. It’s not like it will bring them back from the dead.