I've experimented with Terminus and it's great against Mentor and the rest of the field, until you run into a combo deck; sweepers are very hit or miss in Vintage so you can't just go ahead and start a deck with 4 of them. But if you could, Mentor would be in trouble. (while Yawgmoths Will would be excellent)
This sounds a bit like Legacy where Miracles has to deal with a similar amount of combo decks as well. I could see some variation of it become viable in Vintage as well.
On the topic: couldn't it be a solution to restrict Preordain, Probe, Misstep and Mox Opal to keep Gush and Mentor in the meta? Blue decks would be forced to play better manabases again and could play more not so conditional counters.