The DCEU is getting a sort of soft reboot in late 2022 in the form of The Flash thanks to its multiverse-based premise. Keaton and Affleck's Batmen are both going to reprise their versions of the superhero, with Robert Pattinson's iteration of the Dark Knight in The Batman existing on his own terms. DC Films described it as "Earth-2," and given how Matt Reeves is getting a planned trilogy and an HBO GCPD live-action series, it appears like fans are getting something in between Nolan's universe and the full-blown DCEU.
The GCPD series' announcement made it sound like more supplemental projects are in the pipeline, potentially meaning more characters to populate this world. Should things stay confined to Gotham but gradually introduce more than just Batman, the Gotham Knights could be the next best thing.
Dick Grayson Cameo
Of course, it would be comic book blasphemy if fans get to see a live-action Gotham Knights but still denied the inclusion of Dick Grayson--or Robin in general. Grayson was the original Boy Wonder and one-half of the Dynamic Duo, so a Gotham Knights adaptation should include Batman's right hand.
The Batman is still far enough out to where no one can say firmly one way or another, but perhaps it could introduce him through a cameo scene. Similarly, the GCPD live-action series that'll come afterward could include him in a small role to indicate a future major role. With potentially four Batman projects (trilogy + GCPD) centered around Reeves' universe to be planned, Grayson could be worked in early enough to ease into theatrical sequels as Robin.
Robin In The Batman Sequel
After a concise introduction of the character to eventually take up the mantle, Robin can come into his own in the first sequel to The Batman. In this case, it would be assuming that Dick Grayson appears in the GCPD series. The latter could possibly show him during and after the mob orders the hit on his parents during The Flying Graysons' performance--taking inspiration from Batman: Dark Victory--with Dick becoming Robin partway through a second movie.
Having him be a catalyst for this hypothetical spinoff could be written to be narratively and thematically fitting. In the comics during James Tynion IV's Detective Comics run, Tim Drake was a founding member of the Gotham Knights (as Red Robin), but creative liberties could surely be made in this universe.
Have Catwoman Be A Founding Member
Since comic book movies largely deviate from the source materials and establish their own corners of the universe, Catwoman could be written as a member and founder of the Gotham Knights. She'll be a prominent supporting character in next year's movie, so there would be no stress in reintroducing another character later on.
Selina Kyle could essentially serve a similar role that Kate Kane/Batwoman did in co-leader/founder alongside Batman for the team and train the younger members of the group. When it comes to working from the top down, having Batman and Catwoman as two big pieces together could be enough to get this team rolling down the line.
Azrael As A Reformed Hero
Like in the aforementioned Detective run, Azrael also features as a Gotham Knights member after escaping the clutches of the Order of St. Dumas' brainwashing. Jean-Paul Valley could be introduced as a villain in a sequel to The Batman under control of the cultish St. Dumas--which would continue the theme of corruption--before being broken from their psychological stranglehold.
From there he could get his own spinoff before the team forms. Or his next appearance could simply be in a Knights movie being recruited. Azrael would definitely be a surprise and exciting change of pace in adapting for a major live-action universe, as he's typically only well known within the comics fandom.
Robin Spinoff
Given the lack of Robin on the big screen, he certainly deserves his time in the sun. He's a great character in his own right and complements Batman in every possible way, being his emotional anchor in the process. The last time fans saw him theatrically was in the panned 1997 Batman & Robin, but given the modern power of the comic book genre in movies, it's not out of the question to create a compelling version for these movies.
With a deft hand writing, he could still retain the endearing, charismatic, "beacon of optimism" traits that Grayson especially has. Robin could get his own movie dealing with conflict unique to him while also having the plot partly serve as drumroll to a team-up.
Catwoman Spinoff
Of the characters we know are appearing in this universe--and it's still quite early--Catwoman seems the most likely for a future spin-off movie. She's a popular character, and her brand name should be more than enough to do well in a theatrical solo outing. It's been long enough since the brutally-received 2004 movie that she gets her proper time to shine again.
It's unclear where her moral alignment will go as the franchise continues--she currently seems to be firmly an anti-hero--but that's something that could change with her character arc over this initial movie leading into her own. Since she could potentially be a senior member of Gotham Knights, her movie could also transition her arc into piecing together a team, even if indirectly.
HBO Max Original Series Or Movies
Part of DC Films' plans going forward is to release six movies a year, with four being theatrical and the other two as HBO Max originals. Presumably, the HBO movies will be projects that are smaller in budget and more experimental in nature. The latter point could be a positive, as it would imply a more hands-off approach from executives and allow creatives to get more bold.
While theatrical, Joker could be argued in favor of this since it was essentially an experimental arthouse movie that just happened to star the Joker. These aspects could surely benefit spin-off shows or movies for HBO about the more obscure supporting characters. Aside Azrael, Cassandra Cain/Orphan and Stephanie Brown/Spoiler would be exciting candidates for their own projects.
The Batman & Robin
Another lead-up to Gotham Knights could manifest in the culmination of a possible "The Batman & Robin" theatrical movie. After taking the time for Grayson to come into his own partway through a second movie, a third could have Robin share the title with the Dark Knight and act as a sort of homage/redemption hybrid callback to the maligned '97 movie.
Surely it could finally be done well all these years later and the trilogy-closer could leave the ending open into Gotham's other vigilantes to team up by having the Dynamic Duo decide to and start building the Belfry. This would also let the Batman solo part of the franchise rest again and ration out more focus on lesser known characters.