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And honestly, most of us players probably don't care much about how it looks. On Source it already looked pretty good, but the gameplay is what made it amazing.
It's most likely not an "alternate endings" game. It's only one map long and there weren't any obvious times where you had a choice.
The AI itself wouldn't need to be much smarter, it just needs to be tweaked a little and managed better. Easy AIs could ignore any sounds short of gunshots and need you to move faster to see you in the dark, while hard AIs would have their senses not as limited. Easy AIs could be given equipment that makes them easier to fight for you (if you favor taking out targets from a distance, it could spawn more guards with shotguns), but Hard AIs could adapt to your tactics themselves (so in that case they would have better ranged weapons, smoke grenades to cover themselves, and stay closer to cover).
It doesn't sound great, but I know people who sound a lot like that in real life.
I probably wouldn't be scared by it, but I probably would have to delay the mod if it happened to me.
Even if it's not included in the game, it'll still be possible for mappers to make a custom GAUSS-2 survival map, right?