How would you feel if Gotham (and by extension Pennyworth) were integrated into the Arrowverse's continuity?
I bring this up because of the fact that they weren't acknowledged in Crisis; despite the fact that the other major DC shows from around that time were showcased (even Krypton was going to until that scene got scrapped due to Cameron Cuffe's scheduling conflict), still makes me believe it's part of one of the Earth's histories.
A few things would need to be explain or chalked up as post-Crisis changes though:
1.) Nora Fries is dead in Gotham, but alive in Elseworlds.
- (Post-Crisis change to where she's dead more than likely if it were to fit.)
2.) Two different Cyrus Golds,
- Easy one. The "Cyrus Gold" we see in Arrow could be Sonny Gilzean, Butch's 'nephew' who also changed his name to "Gilzean" when visiting Gotham from Star City. Him studying about Solomon Grundy might be a reference to the idea that "Sonny"/Cyrus might have found out what happened to his uncle.
3.) Ra's al Ghul.
- That's Al-Fatih. The first Ra's mentioned in Arrow season 8. His successor, the Ra's before Matt Nable's Ra's, never killed him, but he lost access to his Lazarus Pit and needed one in Gotham via Hugo Strange. (I could get more into that, but I think this other theory explains my reasoning beyond that, minus the deterioration of the waters part.)
4.) Two different Nyssas.
- Nyssa is a common name in the Middle East, possibly even for daughters of the Demons Head. Although I'd like to think that she became associated with the second in line of her father's successors (Matt Nable) and renamed herself to Athena. I mean, the actresses practically look alike.
Name changing is a common thing although, which happens all the time in comics and a lot already in the Arrowverse, like Jack Napier (formerly "Jeremiah Valeska"), Mabel Cartwright (formerly Victoria Cartwright, mother of Jane Doe), Pamela Isley (formerly Ivy "Pamela" Pepper), etc. Minor things like the Magpies in Gotham and Batwoman could just be like the Aunt of the latter, who inherited the recessive "shiny"/Magpie obsession from a long history of family mental issues. All things that could be easily explained away based on what's already established.
The big issue with Pennyworth though, other than the far darker and mature tone, is that the German Reich still exists but in a stalemated state in the late 60s. Legends of Tomorrow acknowledged that the JSA of Earth-1 (so pre-Crisis) went on to fight Russia's Iron Curtain in the 50s. Thus if Pennyworth would have to carry over somehow, it would need to be in a post-Crisis timeline. Another idea is that it is a side effect of Earth-X merging with Earth-1, and the Germans just had control over what they had in 1940 much longer than originally. (Sounds like a major issue for Alfred to resolve in a future season.)
I'm not integrating anything into the other timeline, just starting a fun discussion.