I dont use KOReader, but if you like the default OS then here's a few:
Kobo Touch Extended enables access to Kobo kePub features. It'll automatically convert your epubs when you send it to your kobo (
https://wiki.mobileread.com/wiki/Kepub).
Kobo Utilities, Adds extra function for working with Kobo eReaders. It saves a backup of your databases, and you can use it to check if the database is corrupted
Kobo Configuration options - You can configure any setting you want. If the header doesn't appear in your "Kobo eReader.config" file, then type it out yourself
The Annotations Calibre Plugin is helpful to back up your annotations without paying for a service (they all became paid services..)
KePub Metadata Reader: A plugin for reading metadata from .kepub files when you add them to your library.
KePub Metadata Writer - A plugin for updating metadata in .kepub files when you send them to your Kobo