To be honest this is a problem that Blizzard clearly has not learned from. Which is “wait and see”. I am of the opinion that whatever Xal’ataths plan is, Dimensius being resurrected was not part of it. Given her reaction to Gallywix “giving” the Dark Heart to the Shadowguard. Now would she capitalize on him being defeated and seemly being forced into the Dark Heart? Absolutely. In fact I prefer that as I love manipulators that can twist something unexpected or unplanned for to their advantage. The problem is that we don’t know exactly what her scheme is. I’m sure the Midnight reveal will explain what it is, but I wish there were hints towards that in TWW. Because all it seems to be is “drain power from X source into macguffin”. Those sources being Galakronds corpse (Dawn of the Infinite), Dalaran (TWW intro) and Voided Beladar (TWW 11.0 max level campaign).
I also hope that Alleria damaging the Dark Heart wasn’t something Xal’atath secretly wanted. Have it be a set back she did not plan for. Which again reinforces her anger towards Gallywix who did fix it but wanted to keep it for himself. As that is what led to it being stolen by the Shadowguard. I do not want another villain where they go “ah yes, this obvious setback was what I was actually planning for” like we had with Zovaal.