Players can create new Keywords by getting enough cards with a prototype version of a Keyword into the game.
How Do I Get a New Keyword Into the Game?
If there are seven cards with a specific Keyword added to the game, it will be officially added by the developers in a minor patch as soon as we get the chance.
The first player-created Keyword to be added to Collective was Lifebond.
What is, and what isn't, a Keyword?
You can find a list of the current Keywords in Collective here.
Hook words are not Keywords
In the above link you can see a handful of "Hook Words." Hook Words are terms which have an entire ability, or series of steps, which happens under a certain condition. For example, "Summon" is a hook word which means "this effect happens when the unit comes into play." Summon, as an example, can be followed by a variety of effects.
Because Hook Words are too general and take a larger amount of engineering time to implement, they are exempt from being added when seven instances have been added to the game. For a new Hook Word to be added to the game, we (the developers) would need to see massive support of similar size to that of existing Hook Words. For reference, at the time of wrting there are about 80-100 cards with Summon in the game. The least used Hook word is Feed, which still has about 20 cards. However, Feed is seen by the developers as a somewhat poorly designed Hook Word, due to difficulty of balancing its effect (if cheap, the word is too strong, and if expensive, it's useless).