Sounds interesting! Like the way you think, but this can come with issues
The new maps were carefully organized and placed in a way where the terrains and climates make sense (notice the stepping stones to and from Lonely Cave and Seaside? Notice how when you travel "north", like to Cherika, the maps get a "colder" feel to them?). If we were to have some sort of contest, we'd have to make sure that they continue to make sense ("the portal must be in this corner of the map" or "include twin peaks over there" etc), which may cause users to lose their map-making freedom. Or, some users may ignore this and that can cause issues as well
If we did fall through with this and a new map was placed in every year, I'm sure people would miss last year's winner map etc and be pretty upset about losing it. Not that there's anything wrong with this, but it could cause more unnecessary drama
With the different themes, there are ideas that there will possibly be snowy maps etc in future updates, if we have any
As I said before, this does sound like a nice idea but I think it could cause lots of issues as time goes on especially when there are already ideas of new maps