In case you can't get enough players/interest for a tournament, you could make it a showmatch.
As I see it there is a lot of interest/anticipation in seeing a high-quality casted match on a 40x40 map, as many people hope it will be an epic clash between battleships and other big toys we rarely see.
But in my experience (as a 1600 global), it will be an eco sim-city snipefest. When any attack takes hours just to get to the enemy base, overinvesting into a mobile army is often just a bad idea. I would love to be shown that I'm wrong, but I have never seen an interesting or engaging match on a 40x40 map.