I'm always a big fan of the "map this shape" challenges, but I would recommend going with something that looks a little less precisely like a mid-scale overland map (the features suggest to me that the main map is somewhere between ten and a thousand miles across and shows an inland sea). I suppose that it would be possible to stretch it to be a cave complex, but it really reads like an overland map outline with the thin rivers.
For me, the point of challenges is about creatively overcoming constraints. Overly-broad or overly-specific challenges tend to get fewer participants precisely because the constraints aren't something that potential participants can see past (too vague and they don't know where to start, and too-specific then it's hard to see something other than the one thing looks like it is). The fun of challenges is seeing how folks creatively solve the same problem. Collaborative projects (map X parts of this overall item) can also be fun, but I'm not too fond of them as monthly challenges because there is now the perceived additional constraint of being harmonious with the surrounding items and it's hard to compare the results.
For "map this shape", I really like relatively simple geometric shapes (which I freely admit is likely due to my limited imagination and poor artistic skills). The nice thing about simple shapes is that it's very clear where they are in the map and folks get a very clear sense of where to start. They give just enough structure to immediately start and everyone will rea something different into them. For example, this shape:
dart.png
Is it a city? A castle? An island? A continent? A room in a dungeon? A plateau? The tattoo on a titan's back that got infected and has spawned its own civilization since the last time the titan moved? Some sort of oversimplified cartoon logo from the chest of some superhero that shows a labeled scar from each major encounter?