Just going to say, I think “New School Essentials” is a brilliant idea. I also don’t think it would take much to create under the OGL. Something pretty close to what we saw in the earliest playtest documents with almost everything being resolved by the ability check mechanic, and proficiency bonus applicable mostly at GM discretion instead of having a codified skill list. Stick to the core 4 races and classes and a very lightweight spell list, no feats. Everything else, from subclasses to additional classes to feats to ability score increases become optional add-ons. You wouldn’t be able to play a B/X style character, an AD&D style character, a 3e style character, and a 4e style character all in the same party, but you could build the ideal d20 ruleset for your group.