Update 13 Review/Impressions
I think this is the best update and brings Idol Manager a huge step closer to feeling like a fully-fleshed/realized game! I'll talk about why I like it, where I could see the elements in it going and where I think the rest of Idol Manager could be filled in.
Scandals
I like the utility of scandal points now, as they kinda just sat there before. There's reasons to want to get rid of them before your next single, now. I got a real shock when I tried to fire a staff worth 3 points and got an event!
Sister Groups
I like so many parts: how they interact with the clique/relationship system, how you decide which group they appeal to and how the formation stats per single factor into how well they appeal, etc. Regarding the appeal, I like how it diversifies the types of idols you have due to the fanbases supporting idols of different groups: For example, I've never had an idol with a majority casual fans until I started a sister group targeting casuals, leading to an interesting scenario where despite one idol of that sister group having the most fans of any idol in all groups, she placed 6th in the election because the single was bought mostly by hardcore fans.
I feel like it's improved the longevity of a playthrough, as when you run out of things to do with your main group, you can just start a sister group and try building them up. But it's not exactly the same playthrough as you've got to juggle twice the amount of activities, leading to more engagement.
Where Sister Groups Could Go
I like the changes done to the game to integrate them (Like sales being affected by fans of idols in the single), and even wish some of them applied to the main group (Like being able to "upgrade" your appeal based off the stats of the songs, which would give leveling up stats a bit more importance.). I think some additions/tweak could be:
1) Aesthetic Differentiation
Like I've said above, aesthetic/UI-wise, some things look too similar when you want to focus on one sister group. I think something simple like picking a color for your new sister group, and then EVERY piece of info having to do with them carrying on that aesthetic would be very helpful. Their singles' tabs, member profiles/portraits, etc.
UI-wise, things like organizing their singles on separate pages from every other group, their singles separately from the main group's in the jobs section, the compact view still respecting sister group division, etc...
2) Tools catered to their addition
Things like being able to audition idols directly into a sister group instead of having to bring them from the main group after hiring them, being able to assign workers/coaches/stylists to auto-train within/find contracts for a specific group instead of the whole franchise...
3) Separate concerts/media
Being able to create sister group-specific shows (this mostly pertains to the "random" and full-group cast options) instead of drawing from the whole franchise, and more importantly, separate concerts where sister groups can have their own concert with a setlist drawn from their singles.
Possible Additions/Changes for the Whole Game
What I think the sister groups do well is that they add additional "cogs" to your idol machine and keep you on your toes trying to manage them. I think features that add these types of moving parts would help increase the longevity of a playthrough. Going from small-ish/subtler ideas to big ones:
Measures of success
Just some fun stats, perhaps some screen showing the fandom growth+sales growth of each group over time, showing the fan growth of individual girls over time, etc. The latter might be especially useful for deciding which girls you want to pay attention to, especially when you might miss the fan growth of girls who are doing dramas/shows but aren't in singles if you aren't checking on them.
Bigger formations/double center recognition
Now that sister groups are a thing, I'd love to be able to bring different demographics from different groups to the main singles, but 15 is a little small for this purpose. Hoping to see this number increase in the future!
More genres/subgenees, ability to tell what's trending
Already know I mentioned this, but I think diversifying some of the appeal combos would be interesting and helpful in single creation.
Idol personalities
Maybe idols can have likes and dislikes? They naturally like certain genres/dance styles/lyrics and will get a boost if they're on a song within that genre, or a hit if it's a style they don't like. Maybe some like being toward the front and others get anxiety being in the spotlight. Maybe an idol doesn't like doing cute/sexy jobs and will ask you to not give her those types of deals.
Achievements
Just some completionist goals, like "Release 48 singles", "sell 1 million copies", "have 100 idols at once", "create 10 groups", "unlock every genre", etc, and lots of them!
Deepening mechanics
Perhaps this would be something only in Normal/Unfair difficulty, but I could see certain mechanics expanded on to provide some challenge: I read up some of the earlier promotion for the game, and I thought one of the interesting concepts was that your group would have relationships with specific business that give out contracts. This could be interesting in where business might return to offer better offers to you upon completion of a successful contract, or might blacklist you/a specific idol for sometime if you break a contract/get in a scandal. Maybe the fame system, which kind of comes off as defunct since buzz is gone and also lacked a function, could be tied into this, where when idols exceed a certain tier of fame, they gain access to a new, higher-quality tier of sponsors/businesses?
The individual handshake, while super helpful for increasing sales, is too overpowered right now imo. The more idols you have, the crazier its bonus gets (I have 3 groups and get a 1000%+ bonus now that i've upgraded it a few times, so it does result in a runaway effect the longer you play.) Maybe a way it can be balanced is by tying it to fame and having some additional depth to it? For example, right now essentially each idol adds the same flat percentage, they contribute the same amount of fan sales. Instead maybe it could be based off the handshake slot system? Each girl is assigned a number of spots, and when they're sold, more can't be bought. If they don't sell out, it's not counted as a sale. This sort of scenario:
Individual Handshake has +10% hardcore appeal per idol (in a best case scenario)
You have five idols, resulting in a best case of 50%. Each idol gets 2 slots consisting of 5%, and they're evenly split between them all by default. You go with the default, but due to a couple of your idols being new/unpopular, you only get a ~30% boost rather than a full 50% (Because realistically no one's gonna go for a handshake with someone unknown, right?) But perhaps your most popular idol has more fans/demand than 2 slots would allow: you can take slots from the girls who won't fill up all their slots and give them to her, boosting you to 50 anyways, at the cost of her stamina being drained more. Inversely, maybe your most popular girl is working on a lot of contracts right now and you want her to conserve her stamina, so you have her sit out the handshake by distributing her slots to other girls who can take them on. The girls spend a bit more stamina, but gain more fans than they might have before, you sell the same amount of handshakes, and your original idol isn't in danger of injury. This is just an idea I had that hits on the management aspect of the game (And I've always wanted an option to allow idols to opt-out of handshakes).
Music videos+albums
Like I wrote in a previous post, I think this is a natural inclusion for the game someday down the line, where the PVs might have their own genres that benefit the song they're associated with, and where you can work on multiple songs at once and tie them together to a single/EP/album that's more effective (but more expensive) than a single song. These would also give you a way to include all your idols and have them gain fans, even if they aren't quite ready to be on an A-side. These b-sides could also be used for concerts and help fulfill those center wishes.
More special events
Kind of anti-climactic for a final idea, but I'd love to see a Rock-Paper-Scissors Tournament event be included in the game! Just gimmicky and fun, maybe with you being able to watch the randomized matches in real-time.
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This really does feel like the strongest update so far, I'm definitely going to be playing it for a while. Thank you for taking everyone's suggestions and feedback into consideration, because you can really feel the game getting stronger with each update!