Yeah. Leveraging materia is a little ambitious in terms of odds, it just works as something that would interest all players so you could have everyone participating to some degree (i.e. always have some basic not-really-hard "challenges" available). You'd still make materia available otherwise, just not to the free materia everywhere extent you do now.

The WT structure exactly, would not work because in order to have ambitious challenges they'd need to be available for more than two weeks and you'd need everyone to have the same ones. I guess they could have Khloe offer a separate list of "bonus stamp" challenges that don't expire and everyone can do, and say they allow you to stock up bonus stamps to put down a 10th, an 11th, etc. up to all 16 if you want. Then so long as SE tends to the three line rewards and maybe adds a little extra spice to them people could be compelled to take on some extra stamp challenges.