I played Left 4 Dead 2 for the first time ever, recently. I was immediately tickled by the way it handles players going AFK and leaving the party. It was near seamless, near-zero interrupted fun whether a player left or just took a break.
Beau Hindman invited me to an online co-op room, which is typical procedure in most online co-op games. Someone creates a room, and then invites friends or makes it public for anyone to join. In L4D2, up to 4 people can play together, with an NPC taking the place of any real player not present.
My first experience with this was half-way through a level. Beau said he'd be right back. After a short spell, his character had the word (idle) appear next to it. It did take quite a few minutes of just sitting there, but suddenly the AI kicked on and it was as if I was the only one playing the game with 3 NPCs. I could then go about playing the game as if I was just playing in single-player. Beau, however, did return and immediately was able to resume control of his character; I imagine by simply moving his mouse or pressing a button.
I would love to see the same mechanic in a party-based, Isometric RPG. I could imagine playing Avernum 4 and having a friend contact me on Skype or by phone, ask him if he wants to jump in and regardless of what I'm doing or where I'm at, he could take control of one of the four party members instantly.
Now in a game like A4, during in-between combat gameplay, it's just a lot of the party automatically moving as one, but it would be interesting to let my would-be friend have complete control over movement and combat phases, with either the ability to go as far away from me as he'd like or within a set area. I would fudge the reality for when a friend would decide to quit playing. If he quits with the character far away from me and the other NPCs, it would just instantly zap that character back to join my party.
Deciding to throw in some extra co-op gameplay wouldn't need to be off the table either. Having a few extra gameplay options to deepen co-op gameplay could be possible without unhinging balance or causing any delineation of a haves-and-have-nots quality of content.
For all I know, some RPGs have attempted this, so if you know of any, please pass along the information. I'd love to learn more about it or discover various other methods.