Chaotic tsundere... *shivers* *Lessons in Love-based PTSD kicks in* and I thought I had dealt with enough back then... *AHEM* Anyway, what a wild ride of thoughts you took us on, Fire. First things first, based on how the story's progressing in Serenity Ch2, I believe 9 episodes is the perfect length. And you'll even spoil people with the multiple choices to fit (most) individual tastes. So I don't think you'll see people complaining about it (much). Same about Last Human, I believe. Overall I see a very successfull and fullfiling end to those two projects.
About the possible stories for the two next games, they're interesting to say the least. Story 1 kind of brings me Yakuza franchise vibes mostly due to "big guy with a big heart" trope, even if there'll be 0 mafia stuff in the story. A nice life sim is always good, sometimes they can teach us a thing or two, even AVN ones.
As for this so called "Viral Affair" I can't say I've ever seen any game like it in the 5 years I've been playing AVNs (didn't play THAT MANY but you get my point... I think). I can think of a few ways the story can be either heartwarming in a slow progression to... I don't know... finding a cure while falling in love with the main girl on the way (maybe), OR turn into a terrifying experience should you give us the option to turn down the feelings of the main girl, unleashing the actual psychotic nightmare a yandere really is hiding under their low-key possessive behavior, added to said deadly virus she carries.
BUT these are thoughts and discussions for the future, I believe. Whatever you decide to do, I know for sure you'll pour your very soul on to it and gives us joy, sadness, tension, the complete joyride just like Serenity (in it's current form) and Last Human.