I'll let Gowi field the majority of the questions here, but I'll put in my own stuff too: there is an overarching plot (group of bad guys, fairly evil, causing lots of problems) but for the most part the adventure will be left to the discretion of the roleplayers. Also I should mention that while the action, magic, and over-the-top nature of things is rooted in the world of shonen action stuff, there's a lot of characterization and emotional involvement beyond the simple: MUST FIGHT THE NEXT BAD GUY mentality of a lot of stories. 

As far as a swamp dwelling witch/magic user; well good news! The major arc of the early story takes place in a huge region of swamps! That fits perfectly. Also I think witches are a-okay for the setting, but Gowi gets final say.