I can think of lots, and am surprised anyone would say otherwise - I guess you must be using a really high bar for "meaningful". An example might be, "You know, I didn't want to say anything in the meeting, but doesn't Greg's data lake proposal sound exactly like the project that failed two years ago?"
Looking back though, I think the important thing is not physical proximity, it's conversation without an agenda. If you only interact during scheduled meetings with a defind purpose, you never engage in work-related small talk: grousing about frustrations, daydreaming about improvements, comparing notes, etc. I think these things are important (not to everyone, I get that some people despise small talk, but to some people). If you only communicate in formal meetings, you can end up with 6 people independently realizing Greg's data lake proposal is doomed to fail, but not speaking up because they each feel like everyone else is on board.
And it hadn't occurred to me before this moment, but the way to get this back without returning to an office seems obvious: pairing and mobbing. That's the only time since I started WFHing that I've had or seen others having these kind of un-directed conversations with coworkers: during downtime in a mob. I need to think about this a bit more...