Secret Six Versus...
So, you want to know which team of heroes or villains I want the Secret Six to face? Honestly, the Doom Patrol would probably have been my first choice. Luckily, I've already been given the chance to enjoy that.
Secondly? I'd like to see the Secret Six go up against the 31st century's Fatal Five. Now, I know that doesn't seem to make a whole lot of sense on its face. After all, the two teams are separated by a thousand years.
But I think it's just the absurdity of it all that makes such a match-up so intriguing for me. Imagine a story where the Six get caught up in some twisted mercenary job -- maybe they're supposed to rob Rip Hunter or something. The end result is their being flung forwards in time.
This would not only give us a great and glorious fight between the Secret Six and the Fatal five, but a chance to throw the Legion into the mix. The result is culture shock at both ends.