Killing a character wouldn't make any difference. Ganondorf has 'died' numerous times and come back. Zelda 'died' in Spirit Tracks and hung around. Link apparently died in OoT somewhere but that just heralded the Downfall/last hero timeline with many more Links. As for it happening in the game itself, I don't recall any nintendo game with a downer ending and i dont think that the death of a playable character for a final act is well suited to games and particularly not to Zelda. The player has just overcome so many obstacles to get the job done only to be defeated in a cutscene to end the game... All that hard work for nothing...
Zelda is also the story of courage and bravery winning through, the death of Link at the end of the game would completely defeat the object. However a Link dying at the beginning of the game and then game then starring a new Link to pick up where he left off would work. WW essentially did that When the Hero of Time didn't return. An immediate version of the WW's story where one Link dies and another takes over in quick succession wouldn't work either though because i don't think more than one Link exists at any one time (save for Hero's Shade and the misc incarnations of previous Links).
The epitome of drastic in Zelda so far seems to be WW, it got rid of the world we knew after so long and replaced it with something new and unexpected, it had a dark backdrop[ and while its ending wasn't a downer it was very sad and heartfelt that ended with Sacrifice.
Drastic in Zelda won't come from story or plot it'll come from design and time, for example had Valley of the Flood been real, then that would have been considered a drastic departure for the series with a technologically advanced Hyrule and something that probably would have played like ninja Gaiden or Prince of Persia.