We already have 24 boards visible to the typical user, with 5 additional sub-boards available for 29 in all. Thus, the bar has to be pretty high to merit adding another one, and since this has significant overlap with the scope of fictional board, I'm not sure it meets that bar. If other people are taking themselves too seriously in that board to enjoy the type of thread you have in mind...well, that's their problem.
If we were going to make an additional fictional board, I'd suggest the distinction be to split off fully fictional regions from those that are proposals for the real world. Fictional countries often contain a good deal of illustrations in them (since they often don't use the US MUTCD), so it's currently slightly unclear whether those kinds of threads make more sense under Fictional or Road-Related Illustrations.