Ngl, I would 100% not be mad at a 911 spinoff centered on Lucy Donato's air rescue team.
And it would be a production nightmare, so it would never happen, but I would LOVE if rather than awkwardly shoehorning 1 or 2 "major crossover events" where core cast from OG appeared on air resue and vice versa, they shared backing cast.
Like, we routinely see the 133 coordinating with the air rescue team, and Mehta pops up every so often. The same extras in police uniforms show up at scenes in both shows to do crowd control type stuff. All the dispatch background folks are the same. While JLH remains exclusively on OG, Linda becomes the default dispatcher for air rescue, and Sue, Josh, and Terry all have small supporting roles on both shows. Maybe every once and a while, we see the 118 trucks show up at a scene but it's C shift so the only person we recognize is Ravi or Julie.
There's random easter eggs buried in radio chatter or news coverage about things that happened on the other show, or characters will make reference to a major city-wide disaster that appeared on the other show. Big events like the tsunami and the blackout would have coordinated episodes that aren't precisely crossovers, but cover the same disaster from different angles.
Nothing that would make someone who only watches one or the other feel like they're missing something, but lots of little details that just make both shows feel like they exist in the same city.
Logistical nightmare? Maybe. But I'd still love to see it.