But I would advocate Lois & Clark, Smallville, the Donnerverse, Burtonverse and Wonder Woman 77 being part of the multiverse, even if it's tiny references
Given the established rules for Arrowverse doppelgangers, I'm uneasy including a universe where Superman and Lois look like Jeremiah Danvers and Queen Rhea of Daxam, or one where Wonder Woman looks like President Marsdin, or one where Clark Kent
doesn't look like Tyler Hoechlin. Also, it'd be hard to explain the chronological difference in when Kal-El was born and Krypton was destroyed.
Granted, incorporating '90
Flash has its problems in that regard -- a generation-earlier Barry Allen (who's a doppelganger of Earth-1 Henry Allen and Earth-3 Jay Garrick) becoming the Flash, as well as Iris West, Mirror Master, and Captain Cold existing earlier and looking different -- but there are enough direct doppelgangers (Tina, Julio, Bellows, Trickster, Prank) that the groundwork has already been laid, and the inconsistencies are few enough in number to be glossed over.
I do not understand thinking the multiverse needs to be consistent in quality, tone, or successes. Have you read DC comics? The DC Universe has always been built on wildly inconsistent tones and styles.
But we're not talking about DC Comics, we're talking about the Arrowverse. A given work of fiction needs to be true to its own ground rules, no matter what other franchises do. Internal consistency is essential to good worldbuilding and storytelling; it doesn't matter if the rules of your universe are different from reality or from other fictional universes, so long as they're logically consistent within themselves. This is especially important in fantasy; since the rules are often unrealistic and arbitrary, the only way to give them verisimilitude is to make them internally logical and apply them consistently. As I said, the Arrowverse has established a clear and consistent set of rules for how doppelgangers work in its reality -- namely, that they look like their counterparts and are the same age, but have just followed a different life path. It wouldn't be right to just cavalierly throw out those rules for the sake of a gimmicky tie-in to past shows. The priorities of fans who want references to the things they like are not at all the same as the priorities of professional storytellers who care about the integrity of their creations. Berlanti Productions certainly does plenty to blur that distinction, but not so much that it critically undermines the latter.
