I have no problem with them making harder bosses, as long as I'm still able to beat them. If I get a nice challenge from a boss, I'll feel satisfied finishing it off, but if it's soooo painfully hard that it takes me a million times, I'll get pissed off very quickly (>_<), but there will still be a bittersweet satisfaction from beating it. Honestly, though, I don't really think a boss necessarily needs to be difficult for me to enjoy. I had a ton of fun fighting the bosses in Twilight Princess (especially with it being my first experience in motion controls), despite the fact that they were a bit on the easy end. Perhaps they could have made them a bit more challenging, but I didn't really realize how easy they were until quite a while later.
In regards to number of dungeons (since that seems to be a popular topic here, too), I'd rather them make fewer dungeons that are all done well rather than many dungeons that aren't very good. Of course many dungeons that are done well would be the ideal - that might take more time and effort, but I hear Skyward Sword has the longest development period of any game in the series so far, so we'll have to see what ends up happening.