The chat idea wouldn't work too well, as it would require a java applet to be run on each page (annoying) so a window can pop up.
Member pruning - great idea.
Banner ad tracking - possibly
Custom avatar generator - maybe, would need more details though - PHP + GD.
Image gallery - will be an add-on, and appear as part of the forum if enabled (will appear like another forum, but with thumbnails instead of description/posts/threads/mods)
I am not sure if there is already something like this but it is kind of like a premium member system, if someone donates to the site they become a premium member and automatically get forum rewards ie, larger avatar/signature, no flood limit, etc.