1) I'd love to figure out how to do custom items, because I don't know how to do it. But until then, if you have any specific models in mind, I can probably add them.
2) No friendly characters, at least not ones that actively participate in the fights. That's one of the things I dislike about later Sam games. The npc sidekicks. If they're too powerful, then they take away the action. If they're not powerful, then they're pointless. In SS2 they could be straight-up in the way and eat your shots. Neutral environmental npcs also feel pointless.
3) The thing is, a lot of the new models for weapons also change the weapon's mechanics. The shotguns are faster with tighter bullet spread, for example. Also the alt fire and stuff like that. Is the model switch going to affect those mechanics as well? I don't even know how to code such a thing.
4) Second episode will be deserty and middle-east inspired. But not Babylon or Egypt, more things like Petra, Wadi Rum, Quseir 'Amra. The fourth episode will be in space, with lots of wacky gravity.
5) I'd like to do new game modes, but I'm not all that invested in those things. If someone wants to do it, that'd be good, but it's unlikely I will bother with it. Edit: Extra deathmatch maps would be sick tho.