Nora Fries previously appeared in the Arrow leg of the "Elseworlds" crossover Elseworlds, Part 2 (2018), which saw the introduction of Batwoman, in which Fries was played by Cassandra Jean Amell, wife of the Green Arrow actor Stephen Amell.
Ryan says that no good thing ever happened at Ace Chemicals. Traditionally in Batman stories this is where a criminal fell into a vat of chemicals and turned into the Joker.
McGregor's Disease is the same fictional affliction that Nora Fries suffered from in the film Batman & Robin (1997). It has also previously appeared in the Arrowverse, with William Tockman/Clock King also suffering from it.