I had a question : is it allowed to use primitive assets (like cube, cylinder, sphere...), and in absolute terms, is a procedurally generated mesh allowed (unity's terrain components) ?

It's a lot of question, but  thanks in advance :-)

The challenge is to try and use pre-made game assets released by me, try to keep to the challenge - that's the fun part!

Have gone through the assets mainly looking at the 3d side of things to see what is there and what ideas get sparked, But I did see that a primitives pack isn't there. It would be nice to have something the the iso prototype pack in 3d, and use the protoptype textures pack as the base. Maybe an idea for you next 3d pack