An Easter egg contained within Lucius Fox's journal on Batwoman hints at the resurrection of Ra's Al Ghul, and his daughter, Talia Al Ghul, having a relationship with Bruce Wayne akin to the original Batman comics. It further suggests that the history of the character may be radically different on Earth-Prime than it was in the Pre-Crisis Arrowverse.
Ra's Al Ghul was originally introduced into the Arrowverse in Arrow Season 3, when he came into conflict with Oliver Queen after Malcolm Merlyn's manipulations forced Oliver into a duel with the leader of the League of Assassins. Oliver earned Ra's respect by surviving the ordeal and was offered a chance to become his heir. Later, Ra's Al Ghul's eldest daughter, Talia, was introduced into the series during Season 5, as one of Oliver Queen's teachers during his five-year exile. It was Talia who suggested that Oliver might be able to balance the conflict between the two halves of his soul by creating a persona and "becoming someone else," effectively creating the identity that would eventually become Green Arrow.
Both Ra's and Talia are mentioned in a paragraph in the diary of Lucius Fox, which has been at the center of events in season 1 of Batwoman. Fox's diary contained the secrets of how to make and unmake his many inventions, including the unique formula that made the BatSuit effectively bulletproof and bomb-proof. The journal was written in a near-unbreakable cipher, but Lucius had a special pair of smart glasses that instantly translated the text when he looked at it. When Alice used the glasses to look at the journal, it briefly revealed a short paragraph that made mention of both Ra's Al Ghul and Talia.
The story in the journal seems to be based on a storyline from the comics called The Resurrection of Ra's Al Ghul, which ran through the Batman comics in late 2007 and early 2008. It speaks of Talia going to Bruce Wayne and giving him a reportedly magical set of armor known as the Suit of Sorrows, which was connected to the Order of St. Dumas that created the assassin Azrael. This suggests a far closer relationship between Talia Al Ghul and Bruce Wayne in the Arrowverse than had been previously reported; one far closer to the star-crossed romance the two enjoyed in the comics.
No date is given for when the Resurrection of Ra's Al Ghul case occurred in the journal. It seems likely it happened sometime before Oliver Queen killed Ra's Al Ghul in Arrow's season 3 finale and well before Thea Queen began destroying every Lazarus Pit on Earth. However, with Crisis on Infinite Earths having reset reality, there's a chance that all their work may have been undone and that Ra's Al Ghul is numbered among those villains who came back from the dead when the world was rebuilt. It's an intriguing possibility and opens the door to Batwoman having her own battle with The Demon's Head in the future, perhaps working with the new League being formed by Talia Al Ghul and Thea Queen?