Numerous lines of code reference Source 2, as uncovered by Valvetime.net, suggesting the new engine is in development and primed for inclusion within Filmmaker at a later date.
"Return an str with the current engine version. If key doesn't doesn't exist, assume 'Source', otherwise invalid -- assume next-gen 'Source 2'," one snippet of the Matrix reads.
Obviously, this is as far from an official confirmation as one could really get, but the evidence is compelling. You can look at the files for yourself by going to ".../SourceFilmmaker/game/sdktools/python/global/lib/site-packages/vproj/vproj.py", or you can read more about the possible Source successor at ValveTime or Eurogamer.