I'd also kinda like to see a more focused scope, having all of the player-characters be in roughly the same place at the same time. If, say, everyone's already a denizen of Gotham City, or a member of the X-Men, or what have you, it might be easier to jump in and start jamming with each other than when everyone has to spend weeks or months establishing their particular lore before venturing out to cross paths. More often than not, that leads to less of a group roleplay and more of a collection of individual fanfics with the occasional crossover.
This has always been my spiel. Open sandbox games just don't work as well as those with linear or limited focus. There are too many options, not enough railroads, and that leads to less interaction and more aimlessness.
One thought I had was a world with three major cities, and GM posts addressing specific cities in addition to whatever character posts those GMs have.
Also, this. When I first advocated for the idea of mini-events years back, and then implemented the system in my own attempt at one of these games, this was basically the idea. Using events to narrow the focus and pull collaborative attention together can work wonders. It just requires 1) a GM team who will consistently set up events, as well as ensure they are regularly updated so as to not drag on with or without player input, and 2) a player base who is willing to engage with the events.
I think a lot of inspiration for that concept can be taken from an old game @Lord Wraith ran on Iwaku nearly a decade ago. Set in a singular, vast city broken into different sectors that each had their own ongoing events and occasional mini-events that sprung up. The premise worked well, although there was some issues when it came to certain player interactions that caused it to fail, but overall the concept can work. That's a game that was relatively short-lived but that I enjoyed a lot due to the structure that was provided.
I think, though, given how easy it is for a single GM running all of these things to burn out, that any such system or similar premise would greatly benefit from having an "ideas man" who creates the storylines for the events and knows when to advance them, and the "producer" who actually gets the posts up and running. Divide and conquer. Share the burden. Valuable collaboration abound.