The current missions are too easy and really have no bearing on the real world and as such don't teach people real skills other than puzzle solving (which I like too, don't get me wrong). I understand if you don't want to encourage illegal things but it would be on the same ground as most of the realistic missions. There are still countless websites which are open to sql injections or rfi, xss, csrf, etc.
Maybe people who post crackmes at the eponymous site could allow their stuff to be hosted, or make new things specifically. Or decade old windows software with copy protection which is now disowned by the author could be targets.
Another interesting thing which would admittedly be harder to enforce is make applications where people need to discern how certain structures or classes are composed.
Reversing is still a major part of hacking even though web hacking is en vogue these days, so I think some more (challenging) missions would be well received.