The difficult third game/album/series film.
Let's be honest - it has a very high probability of being a let down, no matter who writes/directs/performs/creates it.
When you play HL2 these days it is actually a very, very simple game, it's very good, but it hasn't aged well. For me it's too simple now, very linear. I got bored trying to play through it again and the episodes were not as amazing as people state. It is an excellent story with incredible atmosphere, it was one of the best game for years as the time, one of the best games ever. BUT it is a game of it's time. It is NOT a game of the present.
I just don't think anything like that would cut it in todays market. When we eventually, if ever, see HL3 it it'll be something like 10 years since HL2 was released. A lot has changed since then, the demographic has changed, the market has changed, but has the tech changed much, really?
Back in 2005 HL2 presented amazing graphics; the source engine was cutting edge, shader rendering was fairly new tech and the effects were amazing for their time. Plus the physics engine was the best we'd seen, which made it stand out from the rest. It had so much new about it that it couldn't possibly have failed and it built on the reputation of the best game of all time (6-7 years later).
I think they might have left it too late now; and I think they know it. To be fair, the opportunity to release a new HL game-changer just hasn't presented itself to Valve.
I just can't see what they could do with the current/next gen technology that can be so amazingly mindblowing that it is the best thing ever and makes peoples eyes melt and give spontaneous lob-ons. Unless they implement some mind blowing soft-body physics what new stuff can they do?
Advances in user interface etc? I for one don't want to be waving my hands and head around at my PC trying to stack barrels so I can climb up to a window...
Plus a new HL would have to have aim down sights. Not having that just feels weird these days, it's almost like a bit of the immersion is missing. When I replayed HL2 it just felt weird, almost like I was disconnected from the game, simply because I couldn't ADS.
Final note - yes you are not allowed to say bad things about Valve otherwise you get called an attention seeker/hipster/liar/vile creature etc.
JUST IMAGINE IF HL3 WAS A LET DOWN. The internet would implode and their would be riots. The fallout for Valve would be devastating. We all know how fickle and vitriolic gamers can be. Especially when their Gods let them down.