Yes, ratio-wise, it's going to be somewhat 80% story and 20% sex. I can say that due to Angel being Immortal and seen America and the way it changed in the 20th century, she may have some issues along the sex section (i.e. multiple "mortal" partners, her sudden disappearances from her partners).
I was planning to have it run at least six episodes, with each episode adding to the drama and to the dynamic between characters (with secrets being uncovered from their past, being Immortal). I probably want to see somewhat of an epilogue after the story is finished ... and The End after that.
I'm saying there will be probably a maximum of three cases per episode, but depending on the case, some with be finished in a few posts while others will take the whole length of the episode. The first episode I want to plan out will just be an introduction to the characters and what they do for a living. The next episodes will add a bit to their history and maybe something they are searching for, hence the name "A Century Searching." Each investigator should have something they are looking for besides wishing to be mortal again. That said, the episodes may take a supernatural atmosphere to it.
Thank you for mentioning downtime. I like the notion for a player to invite other players to play as cameo-esque guest characters (was that what you were talking about?) to probably flesh out the investigator's personalities and struggles. Though they are Immortal, they go through the same problems as mortals. Also, there is the freedom to add multiple NPCs in the mix.
I'm setting this in the present day, either 2011 or 2012.
Immortals tend to keep to themselves, but the trio of investigators is rare. I believe they should have something they are constantly searching for and that is the commonality between them besides being Immortal. I also believe there should be memories of the history between the investigators, like how did they meet, how did they know about being Immortal, stuff like that. In relations with mortals, Immortals I believe will have to keep it a secret in order not to get ridiculed, but there is the fact that they have to disappear and reappear in a different place due to their inability to age or get hurt.
For posting rate, I'd want it to be at least once a week. I would have to account for time zones and experience between players. Some could be more faster than others. I'd like to keep it balanced in the posting.
The investigators have a friendship between each other, more so than others depending on gender. Girls will tell more secrets to girls than guys, that sort of thing. They should know that everyone who works at the P.I. Agency is Immortal due to a past memory, such as witnessing someone heal a gunshot wound in a week, where it would normally take longer. There might be a certainty when one could sense it through observation or some other method or that the person can tell them, trusting the person he/she will keep it a secret.
Thanks for the questions, hope the details are sufficient. Still, this is just building interest among the forums. I don't expect someone to flesh out a character at once.