So, what i would do if I were handed the keys and told to do whatever I wanted: Cinderella Girl election/Stage for Cinderella but make it Paradox Live.

(If you're not familiar, and I assume you as the asker are not, but also for anyone else who's reading: the thing with The Idolm@ster Cinderella Girls is that there are approaching 200 characters -- some of whom don't have voice actresses yet -- and there's an annual popularity vote that affects various things. Girls who don't have voice actresses yet can have them added if they rank high enough in these elections, since they have to be given a voice actress to participate in the group song. Note that I'm simplifying this a lot!)

So, in other words, how I would do it is:
I would throw a bunch of character concepts at the wall, and let popularity vote sort out which ones actually end up mattering. This could be through a mobile game as with Deresute, or it could just be a general voting campaign; it'd certainly be a good way to use up dope points after BoU or the like.
It wouldn't have to be as intense as Deresute's 190 characters; just think like, five potential aspiring units and whichever one wins a voting campaign gets an invite to the next Paradox Live.

This is kind of what I would do with a mobile game, mostly because I don't want a bunch of core content for the cast we already know + care about to be locked behind a mobile game (since mobile games are ephemeral and inherently have a time limit, are horrible for archival, and frankly just create horrible incentives for the structure and writing), and I think having the focus be on new characters while still being able to influence the main canon through that kind of 'graduation' would be easier to swallow (and wouldn't shut out fans of the losing groups quite as hard, lol), but honestly I could see something like that happening just straightforwardly via Paratora, etc.

In terms of group concepts, I don't really have any off the top of my head! There are certainly subgenres of hiphop they've yet to cover, and popular (or, more realistically, "unpopular but dear to my heart", lol) character tropes that haven't been implemented yet... but I'm also a Touken Ranbu fan and "oh, yeah, we don't have a character like that yet, do we???" is a constant refrain every single time we get a new sword implemented, and TKRB is coming up on 10 years, so there's always things that are super obviously "oh, huh, yeah, we didn't have that yet" in retrospect.

It's an interesting question! And I definitely think it's within the realm of possibility; they already added more groups to the initial 4(+1) going from Stage Battle to Road to Legend, and I can definitely see a world where they add more groups in future.
New characters are always a very easy way to manufacture new conflicts and new story threads, and the structure of these things is always made to be modular and easy to scale in that way. I think part of why they introduced new characters for RtL is because Stage Battle was a round robin, so they wanted to inject more groups into the mix in order to keep the second arc from just being nothing but repeat matchups.

Also, Gcrest have been known to do things with really huge casts, so it's kind of their wheelhouse to have very large casts.

That said, I don't personally expect any waves of new groups to be introduced to Paradox Live in the near future, at least not in the same way as the new groups were added in RtL. (I have a pet theory about how I would add more groups to the franchise, but for some reason I have not been elected god-king yet.)

This is in part because the pace and churn of the series is much slower; releasing three or four CDs a year, you don't burn through character concepts and conflicts and the like as quickly as you do if you're pumping out two mobile game events every month. (I know this is a YMMV thing, but I personally have absolutely no issue with the rate at which they release content for this franchise, lol.)

So, in short: yeah, I can definitely see a world where they add more groups, but I personally wouldn't put down bets on it happening in the immediate future unless my insane pet theory comes true.

so this is an interesting question but i want to disclaim that my ability to really discuss the nitty-gritty of music beyond "what sounds good to the neurons in my brain that solidified at age 15" is pretty limited

I think it's kind of an argument of how cohesive you think they SHOULD be and i have always erred on the side of liking it when they don't sound like each other.

so, like, if you take all of the songs that have multiple full units on them, you're looking at:
-Rap Guerrilla (the original version)
-Rap Guerrilla Reload
-Rise Up (the anime OP)
-Here We Go (Parasute Vol.2)
-29 Verse

And honestly, I've always felt like Rise Up and Here We Go both have the problem of basically just sounding, through their cohesion, like a BAE song that everyone else gets a verse on.
This isn't a knock on either of the the songs themselves (I like Here We Go quite a bit, and I actually don't mind Rise Up as a song on its own, I just hate the anime, lol) but i do think that it turns them into kind of a non-diegetic roll call rather than songs that feel grounded in the universe

I have said this before and I'll say it again: except for the group songs (and arguably the collab songs) every song is at least somewhat grounded as being created in-universe, and the groups do not as a whole hang out with and make music with each other. I can't really imagine akan yatsura, visty, amprule, and cozmez all rapping over the same beat outside of the context of a Roll Call Opening Number, you know?

I think the closest to my personal ideal that the franchise has ever done is the original Rap Guerrilla, because those four verses do sound like they were made by different people while also fitting together pretty well... but by contrast, Rap Guerrilla Reload's back half has always sounded to me like every member of a group project did their parts of the project separately and they're trying to staple them together morning-of. I do like it, don't get me wrong, but it's been hard to escape the knowledge that they are four new verses tacked onto a pre-existing song (and the VISTY > AMPRULE and 1Nm8 > GokuLuck handoffs are both lowkey trainwrecks imo). But that said, I don't think that (for example, and im just choosing two verses i really like so I'm not picking on anything) the TCW and cozmez verses on Rap Guerrilla are thaaaat much more dissimilar than any two group's verses on 29Verse if you actually listen to them.. Certainly the TCW verse vs the AMPRULE verse is just as big a gap as between any two verses in 29 Verse, and because 29 Verse was written with nine groups in mind instead of four-then-nine, i personally think it works a lot better.
so like, in theory, rap guerrilla reload is my personal ideal in that all of the parts ARE very disparate while having their throughlines, but it's been a little held back by the fact that it is a post-hoc addition to an extant song

so what i guess I mean is:
to me, 29 verse is kind of what I'm looking for in a big 9-unit song for this francise-- everyone is matching the same beat (it does in fact keep to 132 BPM the entire time; sync up a metronome to keep track alongside and it's deeply satisfying if you're weird like me WWW) without sounding the same, which is kind of the point when the entire point of the song (and the story arc) is that the groups are all very different, coming from different backgrounds, and have different sounds

The issue of Nayuta's sickness is directly addressed by cozmez' REVOLUTION track.

Kanata: What's the temperature?
Nayuta: Hmm... [sigh] 37.7 degrees.
Kanata: Knew it. You have a fever.
Kanata: Hardly. It's not a big deal. Anyway, who are you calling?
[sfx: phone tapping]
Kanata: We were supposed to make that guest appearance tomorrow. They've got other guests, so I'm gonna cancel.
Nayuta: It's not worth the effort. I just need to sleep it off and I'll be fine.
Kanata: Hm...
Nayuta: I'm just a bit tired. This is just what it's like with me.
Kanata: But... ... that doesn't mean you're operating at 100%, you know? You might go back to how you were before the experiment stuff. If that happens again...

Anniversary countdown art isn't a given; they posted countdown art from the original artists (less
Kuniharu Komiya) for the second anniversary, and year 3 posted the First Note special pages as a countdown and had art from the original artists (again, less Kuniharu Komiya) sequentially following the anniversary, but last year (year 4) the commemorative art was posted sequentially after the anniversary.

tl;dr: There's not a reason per se, it's just not a given that they do a preemptive countdown campaign. (Though as of ~45 minutes ago they have posted a teaser, but that's still not necessarily indicative that they're going to do a campaign before the anniv itself.)

So the literal line in question is:
一生檻の中で飼い殺した

The first part is pretty simple (一生檻の中で -> "My entire life, I'm going to be in a cage[...]") but the 飼い殺し is where I had to make some choices.

So 飼い殺し means, very literally, "to keep something while killing it"; it quite literally means, like, to keep a domestic animal alive past the point of its usefulness, or past the point that its quality of life is declining. It also metaphorically means to like, be keeping someone locked into a pointless job doing menial labor.
Basically, what's being conveyed is idea that he's basically just being left to waste away.

This is a very evocative image, but I think any way of putting it extremely directly in English doesn't have the same impact, especially considering the voice I tend to use for Kenta; he's pretty vulgar and he generally doesn't use five tactful words when two rude ones will do, so "left to waste away" or the like felt way too soft for him and for the way he's speaking in that line.
So that's why I chose to put it as "rotting", because that kind of has some of the same connotations-- of something that's dead or dying, and of just sitting around incapable of doing anything but waste away.