Ah, good question Noel. Made-up abilities are fine as long as they coincide well with the feel of Kingdom Hearts. So, for example, metal manipulation would probably be alright, but something like blood manipulation probably would not be. In addition, if you decide to have a personalized ability such as what you described, please make sure it is in a fledgling state. You shouldn't be Magneto at the start of the RP. Just like all your abilities, personalized abilities will grow stronger as the RP progresses.

EDIT: Also please note I added a new rule regarding time and space manipulation to the OP. Namely, they are not allowed.