How does Source 2 not make it onto this "Market Map"? It's mostly used by Valve and a smaller number of studios than something like Unreal, but it's viable none the less. I suspect it has a much broader user reach than several of the entries on the Open Source end of this map.
https://developer.valvesoftware.com/wiki/Source
Perhaps it doesn't qualify by some arbitrary "next generation" status, but Unreal does?