Do you think there is a reason why some characters get social links while other characters get city life stories? Also since Marian originally got a city life story and then got her own social link, do you think other characters that have only gotten city life stories will be the same? Like one day we will get a Riddle social link or a social link with Wonder’s photography friend?
We know that part of it has to do with character popularity; Kotone and Minami both got proper Synergies added later on because they're very popular characters in China, haha, and they ran that poll for the first anniversary asking which characters should get Synergies added as well.
It's also been stated that 4-star Phantom Idols are supposed to get side stories (city life stories? Is that what Global calls them?), while 5-star Phantom Idols get Synergies, although in practice, they haven't added a new 4-star Idol in well over a year, so that doesn't mean much anymore, haha.
In terms of the original development choices, I'd imagine that it has to do with how long of a story concept they have in mind, the tone/progression of the story, and how popular they think a character will be. To explain what I mean:
"How long of a story concept they have in mind": Chizuko's side story, for instance, is very short; while they could have drawn it out to make it a full Synergy's length, the side story version of it is succinct, and didn't really have the substance to cover a full Synergy. They may have decided to make her a side story character, in part, because of this
"The tone/progression of the story": The office worker's side quest (which... they still haven't added to Global, have they?) is more of a light, unserious story compared to what we usually see in Synergies. I don't say that as a criticism of it! But because of that, it's harder for me to imagine them giving us a Synergy that tells that story. Ergo, I think it works better as a side quest than as a Synergy
"How popular they think a character will be": This one's harder to prove, but considering they want people to pay money to pull their 5-stars, I'd imagine they save the character/story concepts that they think people are more likely to spend money on for their Synergy characters, haha. So if they think someone is likely to be popular, they probably develop that character's story into a full Synergy
One other factor that we know has been involved in the past is rewrites of preexisting Synergies. Supposedly, Toshiya's Synergy was in some of the game's beta tests (I haven't personally seen this, but I've gotten multiple reports that this was the case, and there are some art assets that support this), with Seiji appearing in it similar to Leo in Kayo's Synergy or Yukimi in Yaoling's Synergy. However, after the open beta launched, the developers mentioned they were significantly reworking it, and eventually he had a side story added instead.
This could also be considered the case for Masaki, who was originally the Take-kun that appears in Kiyoshi's Synergy, but was later reworked into his own separate Synergy. Since apparently people complained about the characters featuring exclusively in someone else's Synergy (like Leo and Yukimi), it's likely Masaki was reworked because of that, though they may have also expected him to be more popular.
Speculation aside, addressing the last part of your ask, we should be seeing a Synergy for Haruna someday, because she ranked #4 in that Synergy poll I mentioned, haha. Rikuto Shimizu (the photographer), however, did not rank in the top 10 for that poll, so we don't have any indication that they're working on a Synergy for him at this point. It could happen in the future, potentially, we just don't have any specific reason to believe it will right now.
