OpenSpace3D

This thing is needed to adjust this value to increase the picture quality / performance

I'm not sure it's a good idea to use a plugit for the default value, maybe this parameter should be better in the scene render settings

Or do you think this must be change at runtime ?

after a quick look at Ogre code, the mipmap bias is set on textures units on material creation / load.
Change this value on all materials at runtime should means to update all textures in scene and could be heavy.
What I can do is to manage a default value in our 3D engine on texture creation.

Another better solution should be to have a material editor and be able to change this option for each material.

It is necessary all manually. But I wanted to be able to change the settings both in the game settings.