Crossovers with other properties
I wonder how Iron Man or Thor would work with Star Trek. Especially since Thor is (essentially) immortal, what if it took until the 23rd century for Asgard to start re-encountering Earth?
Superman, Green Lantern, other space-worthy heroes, I wonder how that would work out.
What if kal-el's rocket traveled not through just space, but time? Maybe a temporal rift alters the trajectory, and baby kal lands in 23rd century earth instead of 20th century earth.. hmm..