I have a feeling that the new Construction Set will cost money.
I wouldn't actually mind paying if they put a lot more effort into it and made it more powerful with a built in graphics editor for making and editing meshes and textures and more poweful features for editing large amounts of data etc.
The CS can be a pain sometimes and the amount of third part tools that have to be made to make it more useful is annoying.
If they gave us the power to change everything about the game within reason, then some amazing mods could be made.