About a range of topics:
I would love to see an ancient Sumerian around. That would give us a chance to learn more about ghouls. I have long felt that every civilization developed protective supernatural beings - ghouls in Sumer, half-red vampires in the Aztecs, white court in Etruscans, etc. These beings were often created by well meaning wizards, like the half-reds binding a hunter spirit to a human to give him extra powers. In the case of the reds it was poorly done because if they ever gave in to the temptation they became a threat. But with the ghouls in Sumer they were essentially their civilization's supernatural warriors to counter other civilizations supernatural warriors, and they self-limited their population to what could be sustained by eating the natural dead... until they didn't. I had a character concept in the RPG of a ghoul that had psychometry, which is what the religious leader class of the ghouls had until they were destroyed. When they eat something, they learn from it. "You are what you eat" applies to ghouls too. If they eat too much cow, they become docile. To stay intelligent, they need to eat intelligent people. And if one with psychometry eats enough wizards... boom Sumerian priest-wizard ghoul.
The blackstaff (the person holding the position, not the item) chooses who gets the blackstaff (the position and item) next. I assume it is worked out to happen automatically at his death. I think it is very likely Harry will get the staff. Anyone who thinks that mother winter doesn't know where her staff is, is kidding themselves. She's let them have it for some reason, maybe even so it can come down to Harry at the critical point.
I think Harry's time travel will be incorporeal. He will go back as a spirit, since he did make some comment about how hard it would be to move mass through time at one point. The question is whether he needs to have the blackstaff at the time to protect him from black magic ramifications? I think he will generally stay within the actions we've seen happen in the books and fill some holes we've seen. Who repaired little Chicago - TT Harry. Who helped defent Arctis Tor - TT Harry. Mab was willing to wait for Harry, because she literally knew from Harry that she would succeed in getting Harry. There are little inconsistencies we've seen through the book like where Lindquist lives that I think will supposedly be explained by mess ups Harry makes in the TT. We'll have to see how that works out.
I think the Mirror Mirror book will revolve more around Harry saving/redeeming the MM Harry, rather than killing him. The book series is getting much more seriously into redemption arcs. I doubt MM Harry will live through the redemption though...
I've always thought Eb could have used the blackstaff to blackmagic control a high ranking military office and track Papa Wraith by satellite and then just have dropped a missile on him, but I suspect either Eb didn't think of it, or that he wasn't willing to use that much black magic on a personal vendetta.