There were a lot of problems with Endgame that come from the amount of material and the way they tried to handle it, as well as the casting of Bruce Payne as the villain. It COULD have been a good movie - it had all the elements of a good movie - but it wasn't. There's probably enough story for two movies, but with budget, there can be only one!
Connor's diatribe about wanting out, I can see... heck, the guy's supposedly already won it twice... I'd be miffed too.

But seriously, his want out is understandable given elements of the story... but there are problems with the way those elements are handled in the early part of the movie that make it hard or impossible to connect with that part of the story, so it comes across a little whiny.
And you're right... I don't really consider the alleged Highlander 2 as part of the timeline - trying to makes me convulse. Highlander 3 was actually enjoyable, if not still hard to stomach considering they said he WON in the first one... but I let that slide because I didn't feel bad watching it.
I like Duncan... I like Adrian Paul, actually. So I'm up for it if they're willing to try it. Heck, I'll even go into the first one without the baggage created by Highlander 2 and Endgame. If they botch it... I'll deal with it then.