I came up with a idea while I was half asleep this morning about something that Valve should do with the next installment of Half-Life. Basically you have three campaigns. One you play through as Gordon, one as Alyx and one as Barney. During these campaigns you will come across the other two and tag along with them for awhile, just as you did in Half-Life 2, so it would be single player with a hint of coop thrown in.
An example of what I mean is near the beginning you (Gordon) are at Dr Kleiners labs along with Alyx and Barney. Alyx gets teleported fine while Gordon's teleport experience is not the greatest and it takes him else where. This leaves Barney at Kleiners lab. This would lead to each character going along on their own story line and meeting up later on in the game. There is a part in HL2 later on I believe it's when your going to rescue Alyxs dad. I could well be wrong, but I know which part I'm on about. You are in a control room with some supplies and a couple sentry turrets. You have to hold out against a bunch of combine until Alyx gets there so she can do her computer thing. So depending on how well the person playing Alyx is doing (assuming you are co-oping with buddies instead of playing with the AI) you may have to hold out longer if they are having trouble getting to you.
I would seriously love it if something like this was in the next Half Life game. I came up with the idea as I was thinking about Blue Shift 2 and Opposing Force 2, which I am still waiting on coming out (I can only hope). As BS and OP are both in the same time line as the first actual Half-Life game I thought you know what would be good, is if they took the principle of Half-Life side projects (ie same time line different characters) and added it in to the game making one massive game instead of having a number of smaller games.