How long ago was the last mixed tournament held?
Part IRMG+TE, part RTA. (We can forget about duels, as no one plays them anymore).
Or are some people so averse to RTA, while others dislike TE, that it's not worth it?

Added 6 minutes later
There's an option for a regular Swiss-system tournament, where the winning player cannot use either their own race or the race of the defeated player. Once all races are exhausted, the counter starts over.
Or another option: Players in the next game only play with the race of their opponent in the previous game, but against a different player.
And so on, you can come up with many more ideas.