I would rather suggest an anniversary mapping contest as the common map would be very difficult to organise.
I wouldn't mind an anniversary contest already this year. Say, in November. Why ? To celebrate Thief's 20th anniversary (they grow up so fast *sniffles*). I know TTLG are going to have their own contest, but as a tribute to the inspiration, we might try a mini-contest, for small missions only. Come up with a small enough size limit for the base, and work from there... Newb and pro mappers equally welcomed to give it a try. We'd close the entries and vote on them a week or two before Christmas.
Title suggestion: Tiff-taffin' Twenty Contest 2018
At least it would motivate me to get a move on with making my FM.
Edited by Petike the Taffer, Yesterday, 11:19 AM.