I think that challenges aren't worth very much, especially not for long time players (or newer players who have taken advantage of the ambassador program). The gold is more valuable (to a small extent) right now while quest mode is gone, but it isn't huge thing. So I was thinking that perhaps a new currency earned only from the challenges could be added and a new shop(tab) be added that lets you spend this card to unlock promo cards that you have missed or want extra copies of. Anyone have any other ideas of how challenges could be worth a bit more?

Weekly challenges can also reward things other than gold.  They have done it before for dice and a promo card, so there is no reason it can't happen again someday.

 

In theory, it should also be possible to put out Monthly Challenges, or other complicated challenge chains.  We'll have to see where things go in the coming updates.  It sounds like they have all sorts of Glorious plans for the future.

I'd just be happy if the huge progress banner didn't pop up and cover my hand every time I use an ally/defeat a bane/whatever it is that day.