I'd say money stopped Half-Life 3. Most people agree Half-Life 1 & 2 were "successful" games but nothing when compared to the money Steam and Dota have made. Why invest loads of time and money in to a AAA game that makes mediocre profits when they seem to be get 10 times the pay out with 10% of the human resources required? They made Counter-Strike off the back of HL and they made the effort to reboot that... because it was financially viable.
Also if look at the way Valve operates games are made based on the interest of employees. I'm sure there are plenty at Valve who would have a crack at HL3, and probably have, but nothing has made the cut of going in to full production. They teach their employees to kill their babies, so to speak, and embrace change.
Thirdly, HL1 & 2 aren't really that connected are they? The stories are connected but only because they made them connected. THere's nothing in the gameplay of HL2 that meant it HAD to be a continuation of HL1. A bit like the way Nintendo use Mario in whatever new game style they've created. HL2 could have been a totally new game, with a new protagonist and story.
Still, I'd look forward to a HL3 game. And as others have said, really clever innovative AI would break new ground. There's a reason "zombies" are used a lot in games besides being the de riguer at the moment. STALKER was the last time I remember anyone making a bold change to AI behaviour.
Also if look at the way Valve operates games are made based on the interest of employees. I'm sure there are plenty at Valve who would have a crack at HL3, and probably have, but nothing has made the cut of going in to full production. They teach their employees to kill their babies, so to speak, and embrace change.
Thirdly, HL1 & 2 aren't really that connected are they? The stories are connected but only because they made them connected. THere's nothing in the gameplay of HL2 that meant it HAD to be a continuation of HL1. A bit like the way Nintendo use Mario in whatever new game style they've created. HL2 could have been a totally new game, with a new protagonist and story.
Still, I'd look forward to a HL3 game. And as others have said, really clever innovative AI would break new ground. There's a reason "zombies" are used a lot in games besides being the de riguer at the moment. STALKER was the last time I remember anyone making a bold change to AI behaviour.