Back in the late 1990s, there was a team-up with Hellboy and a few DC characters: Batman, Golden Age Starman, and then present-day Starman (Jack Knight).
If any new Hellboy team-ups outside of the Mignolaverse could be done today, who'd you be interested in seeing?
Two I can think of offhand are:
Etrigan the Demon (from DC)
Conan (Dark Horse currently published his stories)
What about others?