Speaking of Arkham games
I think the perfect Arkham game would have self-contained Metroid style locations like Blackgate, Arkham Asylum, and maybe some unique locations that can be created specifically for the game to add a little extra lore to the world of Batman. Once you step outside of those locations, you are now in the "open world". So, we're basically fusing what Asylum and City did.
I like how Gotham Knights had a populated Gotham City with actual citizens. I think there should be day and night cycles. The city gets more dangerous at night, and this is where you would encounter some of the weirder and more dangerous threats like Man-Bat flying around, the League of Shadows or Court of Owls doing stuff, etc. During the day you generally deal with more run of the mill stuff like stopping robberies or saving citizens.
I would like optional missions popping up where Batman encounters other vigilantes and teams up with them or encounters certain villains like Catwoman and you can try to chase her and capture her. I want optional missions to be as fun and rewarding as the main story stuff and I want them to sort of happen randomly. I think it's lame when you have to go somewhere on the map to trigger a side mission. I want these things to happen organically to make the city feel more alive. Like you're just going about your business and randomly see Catwoman climbing up the side of a building or see Black Canary chasing someone down an alley. Or you randomly get a call from Alfred, "Master Wayne, there seems to be a situation down by the docks"
I'd want multiple playable characters, and the story should change based on who you are using. You can switch characters, but the story is determined by who you initially choose and there will be points in the story where you have to control the character that you started the game with. Different characters will have access to exclusive upgrades. There would also be multiple versions of Robin and Batgirl for players to choose from. You want to be Barbara Gordon? Or Cassandra Cain? If you don't choose Barbara, she might actually appear in the game's story as Oracle.
There should be Boss fights that are on par with the ones in Arkham Origins, which are probably the best ones in the series. I would also want to experiment with boss fights where characters team up, like the one in Arkham Knight where Batman and Robin team up to fight that dude during the mission where they go after Harley Quinn.
No multiplayer crap like that shit in Arkham Origins, though I might design the game to allow for co-op.
I want an evil campaign where you play as a villain and try to take over Gotham. You could play as Talia A Ghul, The Penguin, a Court of Owls Talon doing the bidding of the Court, Harley Quinn, Deathstroke, Lady Shiva, Joker, Bane, Black Mask or an evil version of Red Hood
I'd bring back the shock gloves as a power-up for combat, and I would try to further integrate environmental objects into combat similar to Sleeping Dogs.
There needs to be more tech-based stuff incorporated into gameplay, especially when you're playing as Batgirl. Using Drones, hacking systems or machines to attack enemies, taking down a building's electrical grid so you can do stealth attacks to your heart's content, etc.
A greater focus on legit detective work where players have to actually figure shit out and solve cases. I mentioned a day and night cycle, some cases might take multiple days and nights to actually solve as different events play out.
At some point in the game Gotham suffers some sort of cataclysm. There's been some in the comic books. This will change the open world environment, and the player will have to adapt.
More supernatural elements, particularly if you play as Batwoman. She once had a comic series that focused a lot on that stuff. Would be a great reason to get Constantine and Zatanna to cameo in the game as well
There will be specific missions designed around the various Bat vehicles instead of shoehorning them into everything like Arkham Knight did with the Batmobile.
I'd want secret missions that take place in each character's past, and there would be environments designed exclusively for some of those missions.
There's a crime rating based on your actions throughout the game. For example, if you choose to ignore a lot of random crimes that take place in the city, you'll notice Gotham getting more and more chaotic and dangerous. On the positive side, doing so might open up some hidden scenarios and missions.
There's a fear meter during stealth scenarios. If you raise it the AI of enemies will change. They'll get more erratic, change their patrol routines, start randomly shooting at things and potentially injuring other enemies in the process. The fear meter will encourage players to fuck with enemies a bit and maxing it out before completing a stealth scenario will increase XP earned and players will be given a rating as well for the scenario.
I'd want the game to have a few massive battles with multiple members of the Bat family involved (or possible some other characters as well). You can switch control between them. Basically, the tag team stuff introduced in Arkham Knight taken to the next level.
As you collect Riddler trophies and other collectibles, you can unlock fun cheats. Like one that changes the music during combat to the one from the old Batman show that Adam West starred in.
Or maybe unlock cool characters for use exclusively in the combat/stealth challenge missions, like Terry McGuinnes. Or fun stuff like the sidescrolling beat 'em up level that was in Arkham City.
I'd have one specific mission that involves some members of the Justice Leage as Batman helps them take down some super powered villain (or group of villains) that have come to Gotham.
So many fun things you could do in the Arkham universe. I feel like they barely scratched the surface.