Back when our school switched from seven hour or less long classes to four two hour ones, I accidently got study hall for two hours one year and so I spent a lot of time making what could be considered early 'manga'. It was a bunch of characters I created basically doing parodies of whatever I thought of making fun of at the time, like Sta Trek or Power Rangers. A lot of people in my school became fans cause of that stuff amazingly considering it was all drawn on notebook paper.
(ex: my Power Ranger rip when characters were to shout out the dinosaur names when morphing, my versions said... "Easy On Oven Cleaner! Cherry Pepsi! Belgian Waffles! Soggy Socks!" To this day, friends of mine still remember that joke. O_o)
Continuing, I did a 30 page manga back in my high school art studio class, where I got to do basically anything for the whole semester for a grade. It wasn't the greatest storyline, basically overly inspired by the Resident Evil games (was a big fan at the time) with the U.S. military trying to make some sort of chemical to create a perfect and immortal super-soldier for wars that instead creates zombies and they over run New York City with only my main characters to stop the infestation.
My art teacher went through EVERY page and pointed out what he felt needed more shading or more detailing... It surprised me he even did that.
Then just last year I did another 30 page manga for my college Graphic Design class final, that also ended up being my Drawing final at the same time when my teachers apparently conversed with each other about it. (Saved me some major trouble for on class, doing it for two) It was about a young woman that shared her body with a long dead witch due to a pendant around her neck that she could not remove. Some demons show up to attack her, the witch personality kicks in and makes short work of them and then the the two personalities argue comedically cause while one is controlling the body, the other is still conscious inside.
These past couple months now I've sketched up two possible comics to be sent to publishers. One is a love triangle-comedy story about a college guy whom meets up with a girl he knew from Japan as a child when his military father was stationed there while also dealing with another girl he knows quite well that basically acts like a guy around him, putting him in headlocks and stuff.
The other is about a guy whose archeologist (sp?) mother, worried about someone stealing her finds, sends him an old lamp that turns out to have female genie inside that while wants to grant his every wish, she is not apt at doing so correctly.
Both are between 20-25 pages long. Going kinda by TokyoPop RSOM rules for that.
I'm also attempting to make a few others I've had in my mind for some time...