It will likely be a Greek or Roman name in keeping with tradition. The IAU generally let's the person/group that discovers have an influence in the decision but they're the final say on the name.
With two exceptions*, the names are from Roman mythology. So I'd expect the new planet to get a definitive name from the same template. (Please be Janus. It's the gate of the solar system!)
*Uranus is from Greek mythology, with no good Latin equivalent. Terra is trickier; you could argue that it fits the template for Latin and the Romance languages, but most others simply use local words for soil, without a connection to the goddess. That is also called Tellus to add confusion.
