In Fantastic Beasts, when Queenie probes Newt’s mind for information on Leta, she responds sympathetically to whatever it is she’s seen and points out that Leta was a “taker,” and that Newt needs a giver.
Perhaps Tina will be that giver, but in the meantime, what exactly happened in Newt’s relationship with Leta that prompted Queenie to make that observation in the first place?
It also sounds like Leta was kind of an outsider just like Newt, so there’s definitely more to that story to be explored.
But as David Yates’ comments above suggest, Newt Scamander is clearly still harbouring feeling for Leta Lestrange, whose surname will be familiar to those of you that have seen the Harry Potter films since Bellatrix (a Lestrange by marriage), Rodolphus, and Rabastan Lestrange were all huge supporters of Lord Voldermort.