This may be getting off-topic, but I believe that the answer to JM's question is yes, we do need more keywords, but not because there weren't enough already. As we continue to create more cards, and as we assign existing keywords to them, it becomes increasingly possible to have every single Common and most Uncommon card in our decks share the same keyword. While this does not benefit all keywords equally, it changes the balance in ways that did not exist as recently as half a year ago.
If we're going to continually add new cards to the game, it makes sense that we should be adding new and unique keywords as well. While creating one just to assign existing cards to it doesn't match the reasons I've described, it would be likely in most cases that a proposed keyword would have fitting candidates in the existing card pool.