(These are my opinions and nothing but my opinions)
I think it'd do better as a cartoon or an anime (which amounts to the same thing, really. In fact, in Japan they're both called the same thing), because:
a) Zelda is a setting that works on the fantastical a lot. Stuff like fairies, spells, wacky items, creatures and monsters, etc. would, most likely be done with CGI and CGI usually has a tendency to look fake when compared to live action footage.
Animation is much more free and fluid.
b) A serial instead of a movie allows for more time and more structured parts. Instead of let's say an 1h30 romp through the entire plot we can have episodes of 30 minutes each with more in-depth look at plot.
Considering that you can't actively explore the region like in a videogame, focusing on plot and having time to show the variety of the land visually would probably work better.
A movie split into parts has the problem that the story need to have a beginning, middle and end. So if you do split something, like say OoT and do a movie one from early beginnings to entering the light realm (and showing delapidated Hyrule as the climax, for instance) there's the chance that it'll bomb / not do well enough for the second part. Not to mention voice actors, directing staff, etc. is more likely to change or get involved in accidents during production times and get into more trouble.
Your average Zelda plotline can easily be divied up in let's say 12~13 30 minute episodes for an anime season.
Due to reasons already mentioned, an original story would probably be a better pick than simply fitting a fan favorite like OoT or ALttP into an anime and calling it a day.
I have no ideas or suggestion on voice actors or artists, although I do think that the music and soundtrack should be made in true Zelda style by the usual people.
Link should probably speak, yes. It's hard to do a protagonist well if they can't speak (or even have an internal monologue voiced by someone) - it's not impossible, but hard.
I personally think it should be focused in adventure. Other than SS no game really cared much about romance - at least not with the main character - and much of the Zelda franchise is exploration. Given we're already losing the interactiveness we had from videogames, changing the focus from adventure to something more peripheral could be detracting.
While darker undertones are fine, it's important to remember Nintendo prides themselves in being a more family style company and catering to a more casual crowd, even more so recently. So it most likely would go for light-hearted - that isn't to say darker should be completely ignored. A dark episode or two is fine and appreciated if used correctly.
As to whether it should exist or not ... I can't really say. It doesn't need to, it's not like there's a huge outcry for that, but I can certainly see how many people could and would appreciate that.
It could also introduce more people to the franchise.
