A few ideas to discuss.

[Dreadark]

The Challenge scrolls were fun but were extremely hard for poorer players to get a hold of. I never even got one for almost a year.

I think the weekly challenge system sounds good. There should be a chance to get an easy, medium, or hard challenge like in NTG. But if you can't complete it within a week then you would get a fresh new challenge.

THe 'unlocking shop features with levels' idea sounds good and I would go for that.