At least in the standard universe it is easy to fill up planets that were built carelessly. Sure teraforming is a reasonable answer but would it be possible to pay rubies to teraform a planet? Just an idea. This would not actually raise your teraformer level though because that would boost your building score, rather just add to the base of the planets area.

Just thought it might be nice.

just build the terraformer. -1

Well, around terraformer level 9 or 10 + it gets very expensive.

It's not a bad idea but it should be limited if applied and surely never beyond the max of a planet.
This could be also helpful for home planet.

just make it so that the cost goes up with the field size.