While doing research on the MiG-25, I came across a couple mentions of something called Project 701. Supposedly, it was a planned replacement for the MiG-31, and was being developed during the late 80s (but never was built). I've seen a couple different conceptual designs posted, of which this one seems the most plausible;
Although the planform is more than a bit odd for a design that was supposedly to have had low observability features (also, dorsal intakes on a fighter is wrong to me on a visceral level).
If anybody has any more information on this, please post it here.