An Abigail house, by virtue of Abigail being the antagonist, requires a whole lot of lookalikes for the entire duration of the run. Without Universal knowing the movie is already a success, I don’t see them risking it.
I 100% agree that I think casting would be the biggest hurdle, but I think it's fundamentally different enough that the initial approach would also be different- Abigail can fly.
That allows for plenty of tricks to show "Abigail" without requiring a lookalike- dummies suspended in the air, projection effects, having Abigail as part of a victim costume (similar to Sam+Kreeg in Trick r' Treat). Millie, Eleven, Katherine, Ellie, they're all (mostly) human, so it wouldn't make sense to show them anywhere extravagant or as more than just (mostly) plain kids. But for something like Abigail, where you've got an antagonist with abilities and can be anywhere, now there's the opportunity to make the victims the main scareactors, and with clever lighting and sound you can sell that Abigail was there without any physical presence.
I'm not necessarily sure that incorporating so many tricks in THIS haunt is the way that Universal would want to go, bar any contractual issues, but I could see a lot of ways for a successful Abigail haunt without more than a handful of lookalikes. Maybe I'm just excited, who knows lol.
Anyways, female-centric monsters is very interesting, but I feel like it would have to have a very distinct theme to become more than just "Universal Monsters 2019 but Slightly Different" to the general public. But they did that incredibly well with 2021's Bride of Frankenstein Lives, that trailer and logo really showcased that creative direction incredibly well IMO. I don't think that Universal would pursue this direction without a really solid concept, so I think it's a win/win either way.