Lock and Key aren't really elements. Lock cards generally prevent something from happening (such as disabling an opponent's creature) while Key cards enable things to happen (such as letting you add a card from your deck to your hand or undo the effects of a Lock card).
Nice choice! In fact, some of the strongest cards in the game are part Celestial. I've always leaned more toward Ocean, but maybe that's because the first creature I ever designed for the game was part Ocean. Back when I wrote my YGO-esque short stories nearly 10 years ago, the main character used an Ocean deck.
Cards can actually have up to three types, one for each of the following stats: Origin (determines what the card is weak against), Power (determines what the card is super-effective against), and Terrain (determines the speed modifier).