Take the simplicity of 0E and S&W.
Add attribute modifiers of B/X.
Attribute checks like D20 and C&C per the class.
No skills or feats!
Add many of the classes of D20 3.5. At least 15 classes.
Cleric, Thief, Wizard, Fighter, Barbarian, Druid, Bard, Sorcerer, Assassin, Paladin, Knight, Ranger, Monk, Psionic, and Warlock.
At least 10 races.
Simple spell list at first as I have killed myself in past efforts with a "big ol spell list".
Monsters as simple as 0E and S&W. As many monsters as possible.
Old school art. Self published.