The ink has barely dried on the three-way merger agreement between U.K. law firms CMS Cameron McKenna, Nabarro and Olswang but rumors are already circulating that an American firm may be joining the party.

It would be easy to dismiss any such move as wildly overambitious. Law firm mergers are notoriously hard things to put together—most talks break down long before making it to a partner vote. Actually integrating them is harder still.