Well, Zelda U's map resembles OoT even more closely than TP's map did, Lake Hylia features a bridge over it like the Great Bridge of Hylia in TP, and the Wolf Link amiibo is compatible with it.
Also, when Majora's Mask 3D was being released they had the Happy Mask Salesman post a Q&A on Miiverse and at the end he said this:
“My, my… I’ve certainly been appearing all over the place these past few days! If this keeps going, maybe it wouldn’t be a stretch to expect me to pop up in a new Zelda game, too…? Ho, ho, ho… I’ve got some new masks, too, if you’d like to try them on… If I do make an appearance, I’m sure everyone will experience such happiness! Believe in your strengths… Believe… Ho, ho, ho…”
I would not be surprised if Zelda U is the new game in question. After all, part of the reason they delayed Zelda U was to incorporate ideas they had come up with while working on Majora's Mask 3D. And the best place for a game involving the Happy Mask Salesman would be around OoT. Before or after TP would work. I imagine amiibos will unlock masks in the game.