THEY
"They" is the long-established singular gender-neutral pronoun. Its usage dates back to even before Shakespearean times, Shakespeare himself using it. Here's one example, albeit not in the subject case but the object case:
"'Tis meet that some more audience than a mother, since nature makes
them partial, should o'erhear the speech." — Shakespeare, Hamlet (1599)
And here's another, albeit using the possessive case rather than the subject case:
"There's not a man I meet but doth salute me, as if I were their
well-acquainted friend." — Shakespeare, A Comedy of Errors (1594)
And here's a third, albeit using the reflexive object case rather than the subject case:
"Now leaden slumber with life's strength doth fight; and every one to
rest themselves betake, save thieves, and cares, and troubled minds,
that wake." — Shakespeare, The Rape of Lucrece (1594)
ZE
Now, if you want to be "in line with the times," though, the gender-neutral pronoun being advocated by the LGBTQ community and that is in wide use among gender-fluid, trans, and non-binary people is "ze," or alternately spelled "zie." "Ze" has become prevalent enough that it has been added to standard dictionaries.