So in the theoretical sorcery only MBC, what would you kill with? You get Demonic, Grim, and Imperial Seal for tutors, and you do get Tendrils, but how are you going to storm out with zero accelerants? Dela until you can drain life them for 20? Corrupt? How does this deal with High Tide? Or Goblins?
Theoretically, I was thinking of Promise of Power.
But as you and Jacob Orlove have pointed out, High Tide and Goblins would be very good in the format I initially mentioned. I can't think of any decks that would compete against them off the top of my head, so the "only one card type besides basics" might be a bad format for the side event. At this point I think "Vintage with only access to two card types in addition to basic lands that can tap for any color" would probably be a better format for the side event than my previous idea. Of course, I could be missing something again. I'd love to hear other people's ideas for formats.