I mean, technically we don't even need a specific mode (But a small checkbox that hides modern engrams would be convenient) as you can already hide engrams (and with that the items and structures) so all they would have to do is add the Primitive Plus content into the main game, balance some things out so it fits properly and there we go. Sounds easy, probably takes some work to do of course but it's not impossible and would solve the big elefant in the room "What do we do with this DLC".
Edit: After Ced put out the Feedback thing on Twitter, I really hope people will mention Primitive Plus and Wildcard does something with it. Besides all criticism ARK is still a great game and theres always hope they do something with Primitve Plus in the future.