Endurance mode: Piece of cake, already thought over.

Ship/track editor: It is a possibility, but I'm pretty sure you won't bee seing it in the release of this game. The only way it can be done without requiring excessive knowledge on 3d modeling from the player is having custom parts, like in NFSU to add/replace on a ship. Track editor would work the same way but with buildings and track sections. But it will require a lot of coding, especially for positioning the parts correctly.

Custom soundtrack: It's possible, we have sort of thought on that, but it isn't a priority yet...