If there was a sequel, (which unfortunately I doubt.) there is not very much I could suggest to add. Sure maybe more maps or whatever, generic requests. But they did really well with the whole team work factor of the game. One thing I really would like, would be maybe more backstory. Maybe the survivors could relate to each other, or have real relationships. I mean, I was saddened to hear they removed any lust or love from Zoey and Francis. Or Zoey and anyone, they seem to come from completely different backgrounds. They just don't seem to have proper relationships. We should at least find out how they met. Something else that would be cool is minor character customisation, I don't mean complete facial reconstruction or changing the name to something ridiculous like xxPwnage551xx. I simply mean changing shirts or trousers or even a cap or something, mix and match clothes. And when survivors die, they are still found in those "survivor closets" except when they're rescued, they're differently dressed. Players could set up a "first life" character, a "second life" character and so on. Something else to work on would be playing with the AI. While fun, they aren't perfect, none are. Something that could be developed with the AI would be a system of trust. Say you shoot them in the back constantly, you burn them with molotovs and you aren't playing as part of a team, then the AI isn't going to trust you much. If you save them a few times, kill your fair share, offer pills and medical kits, and help them up from the ground, they'll start to trust their life in you. The effect of this would be that if they didn't trust you, they'd be reluctant to give heal you and or give you pills, they aren't going to go completely out of their way to save you from a hunter or smoker etc. and they might even make a completely un-sorrowful comment when you die. Inversely, if they trust you, they'll willingly heal you and watch your back, they'll make friendly commments and might get flirty if they're of opposite gender, they'll go nearly out of their way to save you from a hunter etc. and they'll be devastated if you die. So, more in-depth characters, more backstory, more relations between survivors (It is afterall a very emotional time) perhaps something like the trust I suggested and maybe more maps. Just some things if Valve ever does make a sequel.
