about 2 years ago - MrFreake_PDX - Direct link
I haven't received a complete list of the free features coming in 3.4 "Cepheus" from the dev team, but these are the things I've been told are included (this list may be out of date and/or incomplete):

New Situations system will be free. (There will be paid Situations content as well however)
Some of the Vassalization Contracts stuff will be free, including the ability to build up to one holding.
Some holdings will be free, some will require other DLC (a good example is the asteroid colony ship test site, which requires Lithoids), and some will require Overlord.
Tributaries, Vassals, Protectorates and the Corporate subject type will remain free (and keep most of their effects from previous versions, from my understanding -- integration will be disabled by default now, however).
The new empire colors, empire icons and backgrounds will be free.
There are tons of Quality of Life improvements (ie. build gateways in vassal space, auto resettlement improvements, etc), bug fixes, AI improvements, and modder improvements, those will also be free.