I think a similar setup to the Olympic standards makes sense.
Remake the rankings to include all the same races that used to work. Require two times, one marathon but the other can be a half. Invite the top X athletes. I'm not that concerned if it is 300 or 500.
Currently, under the WA rankings, here are the last couple men ranked:
297 John Binzak 2:26/2:27
298 Chip Ohara 2:21/2:32
299 Caleb Emberton 2:26/1:08
300 Justin Taylor 2:18/2:35
For the women:
297 Jonah Weeks 2:46/2:48
298 Erin Menefee 2:43/2:51
299 Savannah Berry 2:44/2:50
300 Chelsea Barr 2:46/2:48
For whatever reason, WA has deeper women rankings for the US, so other cutoff points:
400 Krystallanne Curwood 1:18/2:52
500 Carolyn Yang 2:56/2:56
I'd assume adding in more US race results and having a window without COVID would knock a decent chunk off those times, but that gives a decent ballpark.
Having a set number would definitely make planning logistics easier, and that is a factor that the runners tend to ignore. Going by rank instead of a time cutoff would lead to some gamemanship in race selection, but I am ok with that.