"Themed" free champ rotations

Example: Ninja week, sword users week, Vastaya week, ice ice baby week etc.

I know there are not enough full-blood ninjas to fill an entire line-up (only 4 energy users out of 137 champs?) so it might require some creativity as to what we call a ninja but that's just one basic example. Pretty straightforward idea, doesn't actually require any new coding or gameplay tweaks or reworks of any champ.