Definitely an interesting discussion here. Pardon my rambling here as I work out my thoughts + summerize some of the previous replies.
I’m not sure my thoughts on the issues that have been presented. On the one hand, not having a true time limit and having it so anyone can jump in whenever is nice and potentially removes presure and FOMO. On the other hand, I feel that just having multiple threads outside of the Clubs would potentially get less participation. Also, having the Club categories makes for an easy place to look and less scrolling through the parent category. But, there is the search function that I forget exists. Plus, my autism loves categories and lists. 
Personally, I’m way behind on everything. Because I’m in the USA, the Book Club is probably the most difficult because of availablity (time too, but availability is the primary reason). In the TV Club I’ve kind of ignored Classic Who at the moment and dropped off after Series 1 with Modern, though now that I own Series 1-10, I might pick it back up soon. With the Comic Club, I made it through the first Eleventh Doctor Volume. However, while I got the Humble Bundle, reading comics on my laptop is not what I’m used to. The Audio Club is the one I enjoy the most, however, I listen to a lot of other audio and have thus fallen behind.
Am I right in thinking that the Book Club probably will be the club with no interest in adding additional side/alternate clubs and just focusing on the VNAs and NSAs for now?
The Comic Club is currently working through Titan’s output. Looking at @Tian’s schedule (see this post: We should have a comic book club - #9 by Tian), we’ll finish with Titan in April 2026 (unless we extend it to cover whatever’s out of the Fifteenth Doctor run). I can see two options here, start a second Comic Club for DWM or wait until 2026 and potentially go into the DWM comics then.
The TV Club is currently running weekly and alternating between Modern and Classic. The big thing here is whether we’d want either A. a second Club to cover the spin-offs (because it appears the a Club would provide the extra motivation that most people seem to need to actually watch the spin-offs), or B. incorporate the spin-offs into the current TV Club where they’d naturally fall in between seasons of Modern Who.
Finally, with the Audio Club, it seems that everyone wants to start doing other ranges, but the question is whether to start another club for other ranges or find a logical point in the Main Range to take a break and do other ranges. Personally, I think a second audio club would be best because at the current rate, the Audio Club will finish the Main Range in May 2029 and I don’t think we necessarily want to drag that out further by taking breaks for other ranges (I mean I could be wrong, I’m not a telepath). As far what other range a second audio club would start with, that’s an entire discussion on its own.
So in conclusion, I think another Audio Club is potentially the best option, the Book Club and Comic Club are probably best left as they are right now, and how we do spin-offs in the TV Club is entirely up to you guys, I have no preference there.