I think they should call it Nibiru to feed the conspiracy theories.

If Mike Brown finds it, he’ll jump all over naming it, and I’m sure that’s part of his motivation for hunting it so doggedly. He’s like that.

pluto was called planet X until it was discovered

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.

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.