Buy an ad pod during the NCAA Men's Tournament Final. "It's over." Show a bunch of fans packing up gear at the end of the season. Hats, pom poms, posters, all going into boxes. Show them sitting on chairs - moribund - wondering what next. Start to intersperse that with high-speed highlights from MLS - just little flashes of it. 1 second of a Nagbe cut, 1 second of Larin blasting a shot... Then show MLS fans going ape-shit, and pitch something like "It's over, but your passion for sports is not. MLS (sound of ball being pounded as some shot blasts into the net) pick a side." Something like that. So often MLS shies away from competing head to head with the mainstream sports, but this might be an advertising spot worth going after. The visual I'd end with is the one on the MLS App - the MLS shield in the middle, and icons for 20 teams surrounding it like merit badges.
How open are channels to advertising games or shows on channels owned by other companies? None of the channels showing the NCAA Tournament show MLS. ESPN, which had many conference tournaments and has the NIT and Women's NCAA Tournament, obviously shows MLS.