Actually, this is exactly covered in our EULA, which you can find in the about section of the game:
“If you make any content available in or through our Game, including (but not limited to) buildings, building templates, items, and other content, you give us permission to use, copy, modify and adapt that content for any use, including in-game, on our website, and for marketing or other purposes. Your permission is irrevocable; if you do not want to give us such permission, please do not make content available in or through the Game.”
This clause is exactly to protect us from a situation where some mean-hearted person makes a mod of something that they know we’re planning to be in the game, and then sues us for copying them. At the same time, as game creators, we respect the fact that the stuff you built has your soul-essence and creativity infused into it, so we’re careful to only put stuff into the game that we know we created. If we do ask someone from the community to add something they’ve created to the game, as in the case of @Froggy and @RepeatPan 's holiday mods, we make sure to call out who really made the stuff. Thus far, it’s just the holiday mods, and fountains and lights that @relyss made before she joined our team officially, which you can find in startermod_items.
We also ask that you not make mods of stuff you know is on the roadmap, because we think your time would be better spent on non-redundant efforts.