Ugh... so basically you want more content. You can't get more. Devs are working on it but the game will end anyway.

This is a sandbox game, where you can skip parts of it, go to places in different order. Cinematics wouldn't make any sense, since killing the boss doesn't change anything in the world. You just get an item which is required for some part of the progression.

"just ANYTHING to show the player "You're done here, congratulations you beat (this stage of) the game""
There is a thing like this. Placing the trophy inside the Ring.