The Hero of Time definitely deserves a trilogy, though I doubt Nintendo has the time to squeeze in another 3DS game before the successor comes out. Especially when MM3D took three years.
I'm the biggest believer that Link should always be a child, but this has always been the acceptation for me. In OoT, Link was both a child and a child stuck in a young adult's body. In MM, Link got to truly live as a child. If they ever decide to end the trilogy, I'd hope that they'd allow Link to grow up and experience true young adulthood for the first time.
I'd also seriously hope that Link not return to Hyrule. It should take place in yet another different land. That being said, it also shouldn't just be Majora's Mask 2. It should carve out it's own identity separate from MM, just like MM did with OoT.
EDIT:
I actually made a thread about this last year. The reactions were pretty interesting.
http://gamrconnect.vgchartz.com/thread.php?id=183424&page=1
(Just remember not to comment on the thread as it's really old and that would be against the site rules)
