Switching characters has never worked for me because generally I don't like the move sets of everyone else it makes you play as (not including the MM transformations). Also, the 'partner' dungeons in WW made sure that I never played all the way through that game more than once. So, what I WOULDN'T like to see, is more dependence on characters other than Link.

I like Daeth's idea about unlockables, and it kind of made me think about Ura Zelda/OoT Master Quest. It wasn't an unlockable but it's still my favorite bonus feature ever released. I think there should be a system in place like Metroid's where the ending changes based on the percentage of completion you get. That would rock my face off!

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My timeline theory:
Same sword, different day.