Can I destroy a world?
No. The Celestials put a seal on all keyholes before they left.
However, there may be times when mods decide that a world can be destroyed. Only mods can do this.

I madeup a world for my character. Can you make it a board?
Yes! Madeup world boards cost §6,000.

How come you make new worlds, but members can't?
Because these new worlds benefit the site. Only worlds created from staff imaginations will be made (this excludes madeup worlds, as seen above).

Can I claim a world?
No. No one ever owned a world on the Kingdom Hearts games. There's no reason to allow it on MR. In addition, it causes people to become arrogant and god moddish. Then, they try to take control of the site's story. It's happened before.
Besides that, if one person owns one, then everyone will want to. That leads to chaos.