I think you all (with the possible exception of Aerton) misunderstood my main idea: although what you're talking about has to do with the last thing I wrote (well, before the edit), I'm talking about a system to be used for SINGLE PLAYER online play. It has much less to do with multiplayer or game recording, at least for the moment.
If my guess is correct, at the moment there are [relatively] massive amounts of data being transported between server, player and watcher computers, occasionally (or in my case always) causing alot of lag.
If, on the other hand, only the RNG's state and the player's moves are transported between computers, lag will be likely nonexistent.
This should work for the same reason KeyMovies work, where on replay the game is effectively played again, with RNG (which is effectively not so random) giving the same results as it did last time, including level layout, item and monster spawns, damage results and so forth. The game will be played partially on the player's computer (which will require having some game data on it, like the current level), sending moves to the server which duplicates those moves - and therefore the rest of the game. On level travel, new data will be sent to the player (the new level, changes occuring by calculation of turncount etc.), before resuming duplication play. A watcher will be doing mostly the same, though data will only be sent to it - the watcher will recieve level data on connection, and from that point only move data will be recieved (barring level travel), and the game will be duplicated here too. All this may require frequent checksum tests, at least for a while, but also these are minor in size.
Aerton said this requires the game to not have real values in its logic - I haven't looked into the code, but I'm sure this can be managed, even if this will require a [possibly heavy] game mod.
The first things I can think of are magic affinity (at X*1.5*Aff, where X is power up to this calculation and Aff is the number of ranks of this affinity) and ability values (where 17 Int means multiplying power by 1.7). But these can be worked around, assuming they aren't already.
Multiplayer may use this in some way, yes, but for the moment I dare not look that far - I don't even know if this idea, simple in comparison, will work.
While I was writing this Kornel wrote a different idea - sounds bizzare. Ever seen Fight Club? It gives me a "haunted" feeling, like the world around you knows things that you don't. This idea could be incredible, but I suggest making a seperate game which will be designed to contain the unique feel and to augment it.