I think I'd want to do a three week burst. I know after a while we just had a master list of challenges, and you could pick two for each week. The petering out is a problem that I know I deal with!
I'm going to look at some of the old HBC's and see what might work.
I don't know what the HTCI things are like, so I wouldn't be incorporating them. We could add it as an additional part though!