I have had an idea. It has to do with the Supremacy DLC that gives you Behemoths.
There is tech already that allows you to destroy an entire system, and that changes the star type too. There is also a tech that allows behemoths to rebuild planets, making them lava planets from destroyed ones.

The idea I had was for a mod to allow behemoths, with certain tech unlocked, to take one level of mass(planet size) from a destroyed planet and carry it to another system. At which point, you'd have two options:
1. If the system has less than five planets, you can drop the planet mass in orbit to create a fifth planet starting at Tiny.
2. The mass chunk could be added to a planet, raising its size by one level. Adding to an existing planet will make it molten.

You wouldn't even need to leave the system. Have a large planet and a tiny planet? Use the behemoth to take that tiny and add it to the large to make it huge. Though it would have to be destroyed first, and it would be better to canibalize other systems to improve your own.

I envision a system of 5 Huge Atoll type planets.
n_n