If one day Todoist could be integrated into seafile its would be great.
As far as I know the developers doesn’t have any plan to build a so called all-in-one solution. They said more than once that they focus on a rock solid file sync platform which won’t ship features such as Nextcloud’s calendar or todo plugins.
Maybe, but only maybe, we will see something like a plugin API in the future which allows it to integrate own plugins in the web interface or something like that.