Could you please allow more direct access to the server.properties file in some way? While it is possible to change the world generation type before generating a new world, it is seemingly impossible to change it for a preexisting one.
Example: I set up a modded server, one of the mods included being Biomes O Plenty. When the server started up, the generation seemed Vanilla which was fine-- I assumed there would be BOP biomes somewhere further from spawn. Upon adventuring out, and eventually using some commands from the mod itself, I realized that the world was not generating these new biomes. In order to make it do so, I would have to generate an entirely new world with the "BIOMESOP" generation type! If I could access the server.properties file, I could simply change the generation type so that any chunks not yet generated would have the Biomes O Plenty generation instead of Vanilla.
Please consider adding this feature, or at least allow us to directly change the generation type without creating an entirely new world. Thank you. 
tl;dr - I dont want to lose my progress in my vanilla world to add Biomes O Plenty when it could be possible to keep it. Can't do it without being able to change the generation type directly.
