Chloe Sullivan
Here’s a left-field choice. What if the non-comics character coming to Legends of Tomorrow next season is actually Smallville‘s Chloe Sullivan? Clark Kent’s teenage best friend was an invention of the show but she served a hugely important role across its ten seasons, as she functioned as the young Superman’s tech support. Eventually she took on the codename Watchtower, as a nod to the Justice League’s HQ in the comics.
Chloe is absolutely the progenitor of the Arrowverse’s variety of tech support characters. e.g Felicity Smoak, Cisco Ramone and Winn Schott. The Waverider crew don’t actually have their own tech support, someone based in the time machine who can help them when they’re out on the field, so why not go back to the original and add in a new version of Chloe?
Alternatively, Chloe eventually claimed the helmet of Doctor Fate in Smallville, so introducing her could also lead to another DC icon turning up…