tbh I’m very deferred from fics about the next battle because I just want to read fics about the canon characters but SO MANY zb fics on ff.net have the same description about how it’s been 1000 years since the last battle.
however, that’s not to say that a new millennia fic is impossible to get right. if you want to have a good start, I’d recommend starting out in the mamodo world first. too many fics just jump ahead into how their two OCs partnered up from what I’ve gathered from skimming many of these new battle fics before I ended up closing the tab. go into how zatch’s reign shaped how the mamodo world has been run for the past 1000 years and how this affects the upcoming battle. otherwise, it comes off as being so disconnected from the canon that we know and honestly then I wouldn’t care about reading it.
I haven’t created any zb OCs myself yet, but the whole mamodo/book owner mechanic does have a lot of potential for interesting character dynamics. that’s good that you already have a lot of ideas, since you’ll need a lot of them if you’re set on writing this to the end of the battle. as a general rule, just try to avoid borrowing too much from what the team dynamic of what any of the canon teams are like. and try to avoid having characters that are descendants of the canon characters because that’s really cliche.
as for the plot direction, hm that depends on what you’re trying to get done with this fic. zb’s very character driven and a lot of the plot revolves around kiyo and zatch bettering themselves as people as they become stronger along the way in order for them to become the big damn heroes to save the day. what did you have in mind for your main characters?