I don't know how powerful themed teams could be but they would help break up the monotony of seeing the same characters as opponents. Perhaps the lotb coders could give bonuses for creating a team for the arena with differing advantages.
The first team I propose is team 666.
Beast Eddie, the Beast, Wrath, and Beelzebub.
To have the furies keep going off constantly, make sure that they have guard talismans. I also would put burns or thorns. It's not built for power, it's for fun and to meet a theme.
Feel free to post your own themes, such as using krampus coalgiver and Christmas elf and snowmen for example.