I think if doing this I would have a group split so one has to create an app that's finished (defining that would be fun) and one that has to write one that's code is 'clean' (again, fun defining that).

I say this because I've worked with a lot of inexperienced developers that commonly have a stack of half finished projects, or things they've re-written half a dozen times to get the architecture right. I'm my experience you learn a lot by finishing things, and being constrained by previous decisions.

Bonus points if you can get customers relying on your work, nothing motivates like support emails.