Anything I said in this thread is reasonable.
Sylvanas isn't above doing dirty things to get her goals.
Fact.
Sylvanas most important goal is to avoid damnation.
Fact.
Sylvanas needs forsaken to keep her safe.
Fact.
Sylvanas needs the Val'kyr to keep creating forsaken.
Fact.
Sylvanas is willing to make deals with devilish creatures in order to get what she wants.
Fact.
There's a new Lich King which Sylvanas has no personal vendetta against.
Fact.
This new Lich King does have the powers Sylvanas needs.
Fact.
It's possible that Sylvanas and Lich King could make a deal.
<---- Speculation based on facts.
It's pretty reasonable. You're just confusing "reasonable" for "things I like". Just because you don't like a possibility does not mean it can't happen.
Dislike my speculation if you will, but you can't say at all that it's unreasonable.