I have an idea for a structure you can build on Mustafar. In the film Rogue One there is a scene showing Vader lives in a castle on the the planet. He has a Bacta tank, servants and receives official imperial vistors.

In the game there could be a bonus it provides, such as Dark Side user bonus, economic administration bonus, maybe a charge up bonus that Vader can benefit from if he returns and collects after some time. Any idea like that.
You can weigh up deciding if you want Vader on the offensive in the early game or back on Mustafar collecting a benefit to the empire in the late game or powering up before a campaign to take a strong world.

Maybe moving Mustafar to be part of the Eriadu Sector, instead of the Alluyen Sector in Rebellion space, making Mustafar accessible early Imperial games.