I've always been interested in messing around with the setting of Zelda games. I think it would be interesting to see a modern day/futuristic/Victorian-era Zelda. The basic game mechanics could remain more-or-less the same (dungeons, puzzles, temples, combat) but one could maybe introduce different weapons or items based on the era. (Although I still feel that guns would be awkward in a Zelda game.)
Also, finding the entrances to temples in a modern day setting would be kind of fun if it involved searching through sewers, mines, subways and libraries. A little Indiana Jones-in' and I-never-knew-that-church-on-the-corner-was-actually-the-Temple-of-Time might be fun in my Zelda.