Dear Shadow, I rather think that than one week contest (not just dropping box) each week on a different subject and themes is a by far too soliciting stimulus for people usually posting in here.
As a matter of fact, we're most likely not so many and with enough spare time to support such a challenging effort continuously. Morover it's unlikely we're either inspired or going to paint or draw a certain subject in that precise week; we're probably engaged elsewhere.
What You propose are certainly interesting ideas in view of a longer term collection gate as it is mostly the case till now for other threads in this Section.
Generally such project threads were proposed with some examples and starting samples by the initiator to be then let progressively grow along the time.
For instance, when I'll have time and a good enough idea for a dragon I'll post in the relevant thread and in the Fantasy thread too if i feel it to be good for contributing.
I certainly have a couple of dragons or burn blended layer works out of old paintings, but that would be or not in scope for a specific thread whenever it would be either re-discovered or felt appropriate. In my modest opinion, a contest would instead imply a brand new and on purpose effort (let be a jury, a prize etc. not applicable to homemade games) and certainly it can only occur each now and then and a long enough term.
Herein there are also other example of "dry" or reduced attendance Sections due to an excessively specific or local themes and subjects. For instance manga drawings or steampunk or painted lyrics (hard to significantly share and enjoy among so many different languages speaking people) although very interesting cannot be expected to find enough attendance and continuity, I'm afraid, even if they do deserve to be proposed and seen even in a short series.
Therefore, for a very popular thread and proposal, we always have to look for some quite general, shared and continuous enough idea and inspiration which would then still require a long lasting support by who proposed and "maintain" alive such thread.
A couple of great and durable past examples, among the most original series, were for instance Food with an Attitude and The Rabbit's Hole paintings! Have a look and fun by reviewing them!
Panta rei (everything flows)!