Sym-Bionic Titan had romance to it, sure, but it was also a different show with its own tone and style.
Also, with the way Jack and Ashi have been depicted, a romance between them would be creepy. Jack's going to see her as a young kid who's had her life ruined because of him, and that's if he doesn't just end up killing her before finding out. There's not going to be romance, it just doesn't work on multiple levels.
Well okay, what about if we still delved into the complexities of Jack's repressed emotions of not wanting to form anymore social relationships by providing him with daughter fig--
*suddenly remembers all the ridiculous/extremely unneeded Last of Us comparisons in the Logan thread*
You know what? Forget what I said. Carry on.