6) Zatanna
Season four of Arrow has introduced the concept of magic to this corner of the DC Universe, so bringing a character like Zatanna into the quite science heavy team from Legends of Tomorrow would add a fascinating new dynamic (especially in regards to how she’d interact with the resurrected Sara Lance and the mystical Hawkman and Hawkgirl).
[zergpaid]Zatanna is a powerful magician who casts spells by speaking backwards and splits her time between being a stage performer and superhero, and while the possibility of her taking on a leading role in the Justice League Dark movie may reduce the chances of this happening, not including her would be a missed opportunity.