There's a problem with async turns regardless of line-of-sight -- it ruins causality, which is a major component of any multiplayer game.
Take Player A and Player B, not in LOS.
Player A hits the Enemy on turn 1. Enemy flees.
Player B runs into the Enemy on turn 2. He throws a potion at Enemy, which happens to be a potion of gigantism. It lasts 10 turns. Player B and Enemy pillar dance for 8 turns.
In the meantime, Player A gets a cup of coffee, comes back and takes turn 2.
Player B hits the Enemy and it flees back to Player A on turn 11. According to B, Enemy has gigantism for 1 more turn. According to A, it's turn 3.
Now what?
It only ruins causality if you assume that one turn for player 1 is the same amount of game-time as player 2.
I'm saying that implication is flawed. One turn for me is not the same as one turn for you. Turns != Time.
If I'm not in the room with the other player, they could be in the middle of a Rest 1000 cycle. My character could be in the middle of a Rest 1000 cycle. It shouldn't matter.
Of course, it couldn't just be line-of-sight of other players, it needs to be line-of-sight of players or monsters that can see said player.
What it does do is ruin the possibility of having any concept of "game time" so any mechanic which relies on such a concept should really opt for something else -- like having the concept of time be tied to average player level.
Anyone who thinks you can't do real small-scale turn-based multiplayer has clearly never played Civilization with human opponents. Get yourself a copy of Freeciv <http://sourceforge.net/projects/freeciv/> (Mac, Linux, Windows) find yourself some friends and give it a go. By default it's true turn-based and if someone goes for tea, you're stuck waiting. (Though current versions of FreeCiv support 126 concurrent players, that is clearly overkill.)
With Civilization each player makes decisions at the same time. If you finish your turns at the same time there is no waiting for the other player. This approach should work for a Roguelike.
However, it's better if you only need to do this with the line-of-sight limitation I originally mentioned. If I'm playing with someone and that person needs to rest for 200 turns to heal and restore mana, I don't want to be on my own for that time. It's better if I just leave the room and when they come out they're all rested. (Though this has so many repercussions that I think the "rest to recover" logic needs dropped.)
As far as NPCs -- both friend and foe -- there would be two types: those that are only awake when in line-of-sight of players, and those that are like the players and can mysteriously know where the player is at and suddenly be within line-of-sight.
Being turn-based (even the variant where
players do not take turns) should give you very much the same feel as a traditional Roguelike.
Being line-of-sight+turn-based would be a different sort of game. It requires a different concept of time.
The thing is... it's testable within the confines of a 7DRL. Not even multiplayer, as a simple single-player game. It breaks the simple "player goes" / "computer goes" logic that frequently drives simple Roguelikes. Will it feel all that different, though?
More importantly, will it feel more or less like a Roguelike when compared with a real-time game?
Cheers,
Steven Black