possibility to play as minor factions

Thanks anyway ... sh!t happens I guess :-(

 

Is there a plan to make the offline mode moddeable, or to add those missing parts in the editor mode, or even create a civ on our own?

 

https://forums.projectceleste.com/threads/how-to-edit-character-files.4000/#post-6658
The editor itself is still a Work In Progress I believe. You can't edit the civs themselves/mod the game (beyond laying out a map) within the editor though, you have to edit the .char character file the game uses for that civ. Above is a guide Flutters did on editing .char files. There's not any other guides on it to actually walk you through what to edit to get a desired change AFAIK. @Flutters or a dev may be able to help more.