Yeah, there are many things that could possibly ruin the series for me, much of it has already been said, my main concerns would be the following:
- Futurism: I too don't want too many futuristic or modern elements to infiltrate the series, like guns, cars, big corporations, airplanes, modern cities and governments, computers etc. Or going even further than that and having a science fiction/Star Wars like setting: Just imagine Link in a cyber suit, light saber dueling Robot Ganondorf on the moon, ugh :S
Comparison: After playing the NES and SNES Final Fantasy games, I was really shocked when I got thrown into the postmodern world of FFVII and to this day I still don't really like it.
- No more Action Adventure: I don't want the series to have too much of a drastic gameplay change, I want an explorable overworld and non-linear dungeons to stay in future games and not for example some sort of stages which have to be completed successively like in the early Mario games. Or a first person shooter like CoD but with medieval weapons, this is OK though if it's limited to short parts of the game like it has been done in TP with the Old Kakariko Village shootout.
- Ditching the concept of Link: Some people here have already said that they don't want to have a different protagonist, I'm going even further and say that they should leave Link as he is, that means having no preset character (like for example Cloud in FFVII). This allows the player to get absorbed into the game much better instead of "watching" it like a movie from an outside perspective and is a core element of the series. Of course this also excludes talking or voice acting for Link, I wouldn't mind it though if other characters besides Link had the ability to talk.