Currently on the site i have while it is based on a fandom (Sailor Moon), the only canons we allow are the titles roles from the anime. So for instance you'd make an OC and they'd take on that tile of Sailor Venus. While i do love Sailor Moon, I personally feel like the anime accomplished enough. the story is done, and having show canons do things after the show is common. I try to have it be where each group feels include. there are times its still hard because of how closed the groups can be. i consider everyone a canon, so i make plots that include the whole site, something they can mention in a thread. as for how i come up with plots for non canons, and canons, well it depends on whats going on in the story.