Who owns the new artwork can be determined based on the value of the agreement. If the licensee is paying a sufficiently high royalty, the licensee may want to retain ownership of any new artwork. On the other hand, you may feel that the royalty rate does not cover ownership of any new artwork. So in the end, it is a business decision.
Of course, if the new artwork isn't created under a proper agreement, neither party may own the artwork, that right going instead to the artist.