HL3 Will be in the source2 Engine, HL3 Will most likely be the last HL game to play as gordon, there is a high chance of combine victory, Even if the portals are screwed up. Earth is pretty much cosmic forgeted by Xen's wildlife, combine assaults, and bioaugmentation to the advanced soldiers of the combine. Portal will most likely combine with the HL universe here, having Glados or something similar to her being corrupted/Reprogrammed by a combine general, you do puzzles across the lab, fight baddies, discover G-Man's true intentions, and then Die, with gordon muttering his last words, and being his first time speaking ingame, it will shatter the hearts of emotional gamers.
That's my assumption, atleast.
But here's the facts
HL3 WILL HAPPEN.
It will, WILL BE on Source2. It will most likely be Darker then the other HL games.
And besides, we know L4D3 is confirmed/Leaked. HL3 and TF3 are around the corner.