Actually yk what I'm bored here's a rundown of every Nendoroid I can think of with possible relevance to the Cryptonloids so I can wildly speculate on characters that could appear as guests in Starry Party (disclaimer: some most of these are ridiculous and probably won't happen).
Starting with the most obvious, Yowane Haku, Akita Neru, and Kasane Teto.
These three have appeared in Crypton games before and I understand Crypton has at least some partial rights to them so they probably WILL show up. Especially as Haku had her first appearance in Nendoroid quite recently and that feels like it genuinely may have been done with this taken into consideration.
I am very unclear at what stage a Miku derivative stops being Miku and also how the hell did they chibify a character that's already chibi?
Riliane Lucifen d'Autriche and Allen Avadonia.
They borrow much of their appearance from Rin and Len so I can imagine them feasibly being included- at least as costumes- in the future. Possibly depends on the inclusion of relevant MOTHY songs though so idk.
The Project Sekai group leaders.
Technically (?) possible, but for reasons I can't explain feels unlikely. If this were to happen though it may also mean the Miku sekai-specific variants could be included.
Deserves her own mention as she was a guest in Project Mirai, so this would not be her first appearance in a chibi-style Crypton game if she appeared.
Other voice synths- Gakupo, Lily, Luo Tianyi, Zundamon, Ui.
I personally think Lily and Ui are extremely unlikely. Lily because she's been sadly largely forgotten and Ui because whilst she has an inexplicable ability to show up everywhere I feel like she's still very new and a lot of her fanbase comes from vtuber fans rather than voice synth fans. The others I can maybe imagine though.
Black Rock Shooter from the Miku song Black Rock Shooter and the anime Black Rock Shooter. Might need the song Black Rock Shooter added too though as what is Black Rock Shooter without Black Rock Shooter?
Miku appears as a reoccurring character in the anime Jashin-chan is from, Dropkick My Devil! Jashin-chan herself also exists as a speech synth in VOICEPEAK, so she has her connections to the vsynth world.
Touhou and/or iM@s characters.
This list officially enters "silly/don't bet on it" territory from this point on. Vocaloid rose to prominence around the same time as these fandoms and I believe they're collectively known as the big three of Niconico (the Japanese video sharing site). They have a reasonable overlap in fandoms too. Touhou is the more likely one as Miku has collabed with Touhou more recently than Idolm@ster to my knowledge, but both are unlikely.
Miku has collabed with Persona relatively recently and it's very much not the first time. As someone who knows absolutely nothing about Persona, sure why not.
Sega is not involved in this game as far as I know so he probably won't show up.
Okay at this point I'm just taking the piss.
Realistically I only expect Haku, Neru, and Teto though. MAYBE Mikudayo and/or Gumi too.
As an aside, since this is very much marketed as a Nendoroid game it makes me wonder if Nendoroid is gonna use some of its original characters. They might have opportunities where they could appear as part of a crowd and/or as NPCs or something?
I've seemingly maxed out my image limit for this post but here's Emily and Ryo, and I'm pretty sure they have others too.
Also if you've read this far a/ congratulations, I really did not shut up, and b/ please lmk if there are any other obvious Nendoroid candidates I'm missing.