Well in these days the chance you actually make something that really resembles something canon that has not yet been shown in the manga, is quite small. Either way in normal circumstances you can keep it, however you need to take into consideration that your CE/CJ or whatever becomes open to a broader audience, while originally you had all the rights on it.
The example of Bisha has already been said about lava, he can still use it, but so can the characters that used lava. This is also the reason as why custom lava bio's are not yet allowed.
Other example is rubber. It has been a CE For a very long time on NB, but Dodai can use it too now, but only his character.