All of the above are good ideas. I'd like them to clean up some injustices from Elseworlds, notably, the poor treatment of 1990 Flash. When we see cool things like that, and you already have the actor, give him more than a cameo where he gets dismissed like a jobber. They should take notes from the Avengers writers, who juggled a couple of dozen characters and found things for them all to do.
I don't think there should be a big bad either. This year, Cicada was awful.
It was a good season because of the Nora story. Sometimes a hero doesn't need a big bad. Look at Star Trek IV. How heroic were Kirk and crew? They saved the planet, but no big bad--just a probe.
City on the Edge? No big bad. The Visitor? No big bad.
The heart of this season was family.
The Crisis can take place over all four shows, without much crossing over. What if instead of a big bad, next season, all four shows had plotlines related to the Crisis, but not requiring a crossover?
So for example, there could a story on Earth 38 involving Supergirl, and maybe Powergirl, or another version of Supergirl. Or maybe Kara tries to engage on another Earth.
There could be a story on Earth 1 with 1990 Flash and 2019 Flash. Obviously Arrow can show us what Oliver is doing with the Monitor, and so on.
I'm not the writer, but the point is that the Anti-Monitor could be the big bad across the shows, setting off separate stories on each.