I'd see the first Half-Life having a bit of a horror-movie feel, but ultimately being a less-boisterous action movie. I envision a slightly odd way it could be good (it would be quiet for an action movie, for example), but it's not really formulated.
As for Half Life 2, do it from the perspective of a regular resistance member. I think viewing Gordon Freeman's seemingly-miraculous return as a savior from the somewhat-distant viewpoint of his admirers, while focusing on their struggles with the Combine, would be far better than simply retreading his steps from the game. It would be good to have a few scenes as the G-man, however, as long as they're mysterious.
As for Half Life 2, do it from the perspective of a regular resistance member. I think viewing Gordon Freeman's seemingly-miraculous return as a savior from the somewhat-distant viewpoint of his admirers, while focusing on their struggles with the Combine, would be far better than simply retreading his steps from the game. It would be good to have a few scenes as the G-man, however, as long as they're mysterious.