Mav has declared this game to be finished and there does not appear to be any plans to add more content. He is currently busy working on making two other games and the code for this game is not in good condition, so it is not likely that he will add to it any time soon.
A small nitpick, there is no max level for any of the characters, though there is an upper limit on the number of stats they can gain. There are several mods that add addition things to try and some even add a little bit of content to the game.
I don't know his reasons for limiting the story mode races, but I think that he may have felt that they fit the world better. If you want to play story mode as any race, then you can apply the Bugfix mod(https://itch.io/t/1167355/bugfix-for-10d-v6b), restart the game, then apply both the Bugfix mod and the Constants mod, and finally restart again. This will update the Constants mod and add the option in Constants to enable all races for playing story mode. The Constants menu is accessed through the main menu.