Another option is DC's upcoming summer event. Sure, Waid likes his own headcanon, but Batman vs Robin showed that he is not against doing mainline story. I think he also wrote something for Terror Knights? So anyway, Waid wrote a good number of DC's comics in 90s so this event being "30 years in the making" would make more sense with him at the helm than any other DC's top writer. And well, is Williamson going to do another event?
if Ultimate DC is supposed to be more modern then maybe it makes sense to make the mainline a bit more retro to make the distinction more obvious? And I could see Waid being interested in making the entire line a bit closer to his headcanon.
Obviously it can be Darkest Zero Hour by Jurgens, but I hope that DC isn't that stupid.
Edit: Like, this thing also happened in 1994 https://en.wikipedia.org/wiki/End_of_an_Era_(comics)