All games are essentially sandbox games of some sort. All games consist at bottom of some set of more or less absolute rules creating an environment in which the player or players interact with the game entities in some way. There may be an ending, there may be several endings, there may be a 'win' state. Or not. It doesn't matter. The game environment may require a computer or console to be created. Or not. It doesn't matter. The question is always and only - is the interaction with the game worth the player's time?
This is obviously highly subjective - not everyone enjoys chess, CoD, Go, Nethack, Monopoly, GTA, badminton, Galatea, battleships, Minecraft or whatever. But those that do, do. It's possible but actually quite difficult to create a game that is truly linear, but why bother? It's always possible for the gamer to create a metagame within the existing rules. Arguably, the set of metagames created by the game rules *is* the game. It isn't always what the game creator might expect. Nor should it be.
Outside the computing environment, rules are much more mutable: gamers routinely bend or alter them as they see fit for the sake of a better or even different game. Quite apart from variants like Suicide Chess, chessplayers have for centuries evened the stakes between much stronger and much weaker players by having the stronger player start with fewer pieces, or be required to wear a blindfold; Go players have a similar mechanism. But even without altering the ostensible rules, the main metagame in both Go and chess is about delving into and memorising as much as possible of the wealth of existing analysis - learning openings, middlegame strategies and endings. That's not in the rules anywhere. But it's where the game is.
Exploring the set of metagames created by the game rules is the definition of gameplay. When playing a racing game I'll always at some point try and go off-track or go round the course backwards, firstly to see if I can (many earlier racers simply wouldn't let you) and secondly to see what happens when I do. The Nethack DevTeam is well aware of this - this is why they Think Of Everything (do they? Really? Go find out...) and this is why Nethack is so enduringly good. This Pacifist Doom video is a testament to what a good game that was. See also any number of speedruns, or things like Freeman's Mind, or the entirety of machinima - here the metagame becomes 'watching other people play the game'. This kind of thing predates computers - serious chess and Go players use notation to play through sets of games played by others going back centuries.
Game creators who aren't thinking about the set of metagames created by their game rules might make a good game. But only by accident.
originally a comment on the thread discussing Chmielarz's post at Metafilter