I would like the forum to be as lightweight as possible and have a very strong focus on templating and flexibility.
Amen to this.
If you make it so that it can either be integrated as an adjunct to articles like threaded commenting, or like a more traditional forum, that would get my vote. RSS, for sure. And maybe some sort of ping system for updates? I know that sounds more bloggy, but I’m thinking this can be used both directions. And try to leave the frontend templating to us appearance obsessed types, which I’m sure you will, since ModX has been that way from the beginning.
Personally though I know most people are throwing out a total wish list I think that fine line between having a good amount of functionality and staying lightweight and fast loading as possible is probably where the focus should be. A hard line to walk, I think. But folks who want more will be able to add it, since that’s what ModX is all about. Modularity would be a good thing. A "basic package" and then additional stuff as add-ons. MaxiGallery did this in a smaller way with the actual gallery, and then a second snippet to add thumbs to the sidebar throughout a site.