challenge suggestion - building on previous maps
not as obvious as most challenge suggestions, but bear with me...
I have 2 different ideas for this, either 1) as long-term challenge that goes on for maybe 4-6 months, with each challenge building on the last. for instance, the first month can be create a world, the second month can be zoom in on a continent, the third can be detail a region in the previous continent and the 4th can be anything as long as its based in one of the previous entries.
the second idea can be to pick a previous entry (you need to have entered before) and either evolve the map or do a later version in a different style. you can take into account things like increased/decreased water levels, changed borders, some great magical calamity etc.