"A game designer from a parallel dimension with no D&D nor Gygax is tasked with designing a game supporting common OSR principles. What would such a game look like?"The original intent of the question was to remove deadweight mechanics form the game. Ideas that started with Gygax and early versions of D&D, that no longer make sense, but that have stuck around because of their historical inertia.
As an example, clerics as PCs and wizards who can't heal. It would make sense for wizards to have healing magic, and the only reason they don't seems to be to stop them from intruding on the cleric's space. But why is there a cleric in the adventure to begin with? Because one of Arneson's friends wanted to play Van Helsing from the Hammer movies while they were fighting a vampire named Sir Fang (yes, that was really the vampire's name) . That's why there is a cleric class in D&D, and it continues to exist because of inertia.
Remove that history and re-make the game again, and you'd probably drop clerics and give wizards healing spells.
But I was actually more intrigued by the literal wording of the prompt. A parallel dimension with neither D&D nor Gary Gygax. What is this dimension, and what kind of game can come out of it? Let's imagine a universe where we are making the first roleplaying game, where D&D has never existed, and where Gygax also doesn't exist. What does the first roleplaying game out of this universe look like?