We decided to do something special with the local club - instead of a tournament, we would do a small force campaign day where players would try to find their way out of the Realm of Chaos, all the while being mocked by the Dark Gods. We had a good variety of games, with single characters fighting arena style in ruined cities and Chaos strongholds, daemons being summoned to fight alongside (and, in some cases, replace) forces, and Realmgates/portals being used to bounce from one table to another in a campaign-wide treasure hunt to uncover a lost relic that would allow them to escape.
It was seriously chaotic, but a lot of fun and (I think) something a bit different from normal
40k events.
We did a write up of the event here (with piccies):
https://ttgamingdiary.wordpress.com/2018/08/28/event-report-laughter-of-the-dark-gods/