Alright, so. I think the easiest way to solve lower activity rates is to inspire more interaction opportunities. And the most effective way to do that, without arbitrarily forcing people to start writing collaborative posts together, is to incorporate linear story elements into the RP. 

My suggestion, as I've laid out in the Discord, is to utilize mini-events or "sidequests." As has been the status quo over the last several years, these games now tend to have end-of-season GM events that are wide-spanning crossovers able to pull in every player and character at once. As was witnessed in the previous two games, these are very successful and positively viewed. And they tend to bring together players who may not have ever written together in that particular RP. I say we take that concept and miniaturize it down. 

A sidequest would be relatively simple: one of the GMs puts up an IC post detailing some sort of natural crisis or criminal act in a section of the world some of our characters would be close by to. For instance, half-a-dozen high-skilled individuals breaking into a California facility where weapons-grade plutonium is held. That GM would then post in the OOC announcing that this is a mini-event and that any player who can reasonably get their character there to react to this crime-in-progress may choose to do so, and would list a small checklist of what would likely need to be achieved in order to resolve this sidequest. For example: prevent the theft of plutonium, with a bonus objective of apprehending those responsible. The players that choose to respond can then resolve this sidequest as they see fit without further GM action. If multiple players respond, they now have an easy opportunity for their characters to interact. If only one player responds, that's fine, too, as the sidequest still serves to make the world we're writing in feel more lived-in. 

If no one responds, though, after say a week or two, then the GM will then write a second, closing post in the IC detailing the success of the criminal's plans. No one resolving the mini-event would mean "consequences" in that civilian life could be lost, or the criminals could now be even more dangerous the next time. In the same example as above, the criminals now have weapons-grade plutonium and could return in several weeks now holding the city of Los Angeles hostage by the threat of a dirty bomb - thus upping the stakes for a future sidequest. These are the sorts of "consequences" that make the world feel lived-in and real - it's not just what our characters do that build the world, but also what they don't do - while at the same time not feeling like a punishment for choosing to ignore a particular sidequest.

This method, I think, should spur a lot of interaction with minimal effort - GMs would only need to write a single, relatively brief IC post for each sidequest, or two if it is not resolved. It would also help expand upon the world we write in, filling out the backdrop events and giving more opportunity to include these sorts of stories that often don't make sense in our solo arcs. And, further down the road when it makes sense for a year one story, these sidequests can be used to introduce minor comic book villains that may not be utilized otherwise. And, they can even be used to tie-in to the overall season-ending event, whether directly or indirectly, so we can truly feel like said major event has been built up to. There's a lot that can be done with this method that doesn't require any drastic change.

As someone who went into this RP with the intent to fully interact with others this time, but can't always rationalize teaming up with certain characters under normal circumstances, these sidequests could be very beneficial.

Introducing a new sidequest every two weeks or so could be a great way to improve activity, inspire inter-player interaction, and create new, interesting story beats that feel even more like the comic books we all love and enjoy.