Fan creations could be released as monthly packs or individual items in the store.
Allowing them to be earnable as well as purchasable similar to warframe would be huge. This could even be a way to move forward with challenges alongside a progression system. Think MCC points.
I will include a tone of great work in this thread. These are not mine.
A current idea which requires likely the least amount of work is the rakshasa core. Allowing players to submit custom and kitbashed models for the core not only works look wise but canon as well. 👇