I would go for something different than another 'Kingdom Hearts [...]'
Why not go for something like just:
- Final Mix.com (aiming at the name of the complete versions of all the games)
- Sunset Horizons.com
- Xenahort.com
If there somehow would be a site already with that exact name, you could always use a '-' in it, like Final-Mix.com or something like that.
I dont know, Im sick and tired of all the kingdomheartsthis.net and kingdomheartsthat.com.