General idea would be a ROM is picked (randomly, contest, list of top games... such things can be variable and decided later) and then we have say a race to get certain milestones, or just do it for fun. Tables, stats lists, level location, basic level format, binary (where applicable -- you can find a GBA one in 5 seconds and the DS has it in the header). Real rather than cooked up equivalent of a "hackme" would be my preference but... yeah.
Could be a for the prestige, for fun, if the site wanted to kick a year of premium then could do that as a minor thing.
If one wanted to take some footage of it then it could also satisfy (at least until it becomes apparent how tedious it will be to sit through) those that think video learning is the way forward for ROM hacking -- so many things it is, or at least is very very useful, can't even get close for ROM hacking. More generally some screenshots and explanation along the way might be a nice thing as a secondary goal.