Pretty much what LynnJuniper said, with the following clarification:
Anything invented must be within the technology level of the world now, which is not very high, which include things like tools, means of manufacturing, material quality, etc. Anything invented must be within the character's ability to create, based on things like crafting levels, magic caster levels, Intelligence, Dexterity, etc. In all cases, items must be RP-only and not really give an advantage of any kind to the character. Unfortunately, there is almost no possibility of inventions becoming in-game objects, due to us hitting a limit on our palette.