I like what you said up there about focusing on character development and group events! I much prefer that over heavy group plotting centered around the upside down, but I know that's not the same for everyone and am fine to play in either.
As for OC/fandom characters, maybe they should have some ties into an already existing character? Like Eddie having a secret sibling that comes in as OC. And now that I've said it out loud, I wouldn't actually mind that lol. Anyway, it just might help bridge the gab so no one feels left on the outs when it comes to bringing in a lesser known character.