Not a new concept or idea, but:
HL 3 and Portal 3 in Source 2
My new pet theory is that CS:2 came out so that Valve had all their IP sitting at 2. So then at some point, when their audience is too old to play games anymore, and the youth don’t even know that Valve ever made games, they’ll release a 3-box with HL3, Portal 3, LFD 3, CS:3, TF3, and DOTA 3.