I'm sure it would be viable to print a pack with new Heralds, Ancient Ones, Investigators and so on.
How many would you buy for a few new investigators (or not, because of the needed tokens), Heralds for each unheralded ancient one, some Ancient Ones (or maybe those coming from the Arkham events…), some new Gardians (because 3 is small), a lot more institutions (Governement, Newspapers, DeltaGreen… and so on), all scenarios from the Arkham event and campaigns… I think all the fans would buy this to have a complete collection !
2nd idea, on BoardGameGeeks, some though of standalones, because Arkham Horror don't need more new game mechanics to track for, for themes maybe in Dreamland, Kadath or At the Mountains of Madness… Maybe with an option to play in parallel with Arkham for true madness… Or a London or Paris expension…
Last idea, a big case. Look, all of us have storage problems. The only problem is some wouldn't care because they need some moar game (and not minis or case).
This is a successfull game, the problem is if FFG want to sell, they need to find an "horizontal way" or else it will transform the franchise as what a dice and paper RPG can sell : that's why some cheap-to-produce bigs cardbox decks that deserve maybe all game configurations, or a standalone (as an appeal product and/or expension) would be great.