Another note, after giving this some thought, I would like to return back the ability to tell AGS to write files into game directory.
Normally this should be advised against, because users may run the game from somewhere where writing is forbidden (either by system permissions, or physically). But I found on my own that there may be at least one case where you may need this ability: when you have a "development" mode in your game to create game data right from inside the game. Currently you'd have to temporarily set custom "APPDATADIR" path in config for the development time, and reset it later before distribution.
To write to game's directory one would have to explicitly use $INSTALLDIR$ token in the file path. File paths without any token will be rerouted to $APPDATADIR$ as they are currently. Manual must contain a warning that one should not write to $INSTALLDIR$ without serious need, because that may prevent some users from running the game normally.
Writing into $INSTALLDIR$ won't have same meaning as writing to $APPDATADIR$ or $SAVEGAMEDIR$ even if these are set to game directory in config, because, obviously, one may always remap the last two onto another location, while $INSTALLDIR$ is always $INSTALLDIR$, so to speak.
