Well they could definitely upscale the 3D games. I could see that being very possible.
However, I don't see it being a collection, but rather individually sold games. That way that price of $70.00 that it would cost would seem much better...and reality it would be probably closer to $80.00.
The biggest problem is recreating LttP in HD. As it would require completely redoing the games art assets. Boy would I love for Nintendo to take the time to do that. In fact...if Nintendo would do that and use HD sprites for LttP, Zelda 2 (making the combat better) Link's Awakening, and online 4 swords adventure and battle mode...I would spend full price for that game. $39.99 easy. I guess you could add in the original Zelda too...with a proper and reworked over world with more areas to explore and less screen copying.