Ohno and Ogiue are inspired by Hiromu Arakawa.Each of them got different qualities that make them like Arakawa.
The First President of the Genshiken is an alienHe was sent by his species to study Earthlings. So he decided to set up a club dedicated to a popular Earthling interest. Genshiken is in and ends up joining it.
Ohno is an older Kiri KomoriSeriously, she looks exactly like her, except a few years older. Presumably, Harumi introduced her to yaoi, and notably, Harumi's pairings are often from the different Gundam series, which are Ohno's favorite. Kiri decided to put her days as a hikikomori behind her, and thus decided to change her name.
Ogiue is an older Yukimura KanauShe is surrounded by the creative influences (several mangakas) and the darker influences required for fuyoshi-dom (Loli-Nicchi). Suppose she went to high school in the Touhou region and picked up the local dialect. It's a perfect fit!
Katou from the Manga club is female version of Eddie Sukenari from Kimi Wa PettoTall. Dark stringy hair. Creepy looking. Total otaku. And there is the same gag with obscured eyes that are shown sparkley beautiful.
Madarame will murder Kousaka on the day of his weddingHe will then partake of his blood and become a vampire. He will then try to comfort Saki, but it will not work and she will commit suicide. He will then be granted his own domain, a twisted version of Shiou University with the clubroom as his sanctum.
Spotted Flower takes place several years after GenshikenThe unnamed husband and wife are Madarame and Saki. The unnamed wife mentions that her old boyfriend was an Otaku who is also a friend of the husband's, just like Kousaka. The husband mentions that he threw away some S&M DVDs because he could not get into them, just like Madarame. These two are clearly Madarame and Saki a few years down the line.
The First President is already deadThe First President is already dead (NO, Kenshiro wasn't the perp!) and is guarding the club he founded (That explains the stuff from back in '87.) as a guardian spirit. Through his omnipresence (restricted to the university grounds) he's able to watch over any activities that could endanger the club. He didn't put out that fire, because the development set in motion through that incident would eventually strengthen the club spirit rather then end the club.
Genshiken will undergo a Genre ShiftIt will eventually turn into a Horror manga dealing with Madarame's obsession with Saki and how he does not take her rejection too well.
Everything that happened after Sasahara went to sleep in the middle of episode 12 was a dream