7) The Arrow
No, not that Arrow. The latest episode of The Flash featured an Earth-2 flashback which revealed that Robert Queen became The Arrow in that reality, which is an interesting new dynamic for the vigilante that would be very interesting to explore in Legends of Tomorrow.
Of course, that same flashback revolved around him being arrested, so the team would need to probably break him out of jail. Including the Earth-1 Oliver here as a guest-star would be cool as he would (sort of) be reunited with his father, but that’s not a necessity. Instead, someone like Sara explaining to Robert that his son is alive and a superhero on another world would offer some great emotional material to explore.