For "realistic" (though still unlikely) megaton exclusives...
Fallout 4 for XBox One.
Why could it happen? Bethesda has a relationship with Microsoft, with Morrowind and Oblivion initially being XBox exclusives (Morrowind never seeing release on another console, though Oblivion was later released on PS3). Fallout 3 was also generally a better experience on the 360 than PS3, and it seemed like Bethesda developed primarily for that and PC. If I remember correctly, Skyrim was much buggier on PS3 than 360 as well and it seemed like Bethesda ignored the issues in that game for a while.
Why would it be megaton? Fallout 3 was one of the biggest games of last gen. It still has a huge following and it's been rumored to be in development for years.
Why won't it happen? It's stupid to release a huge game like this as an exclusive.