Most classic turn-based games, from chess all the way to Civilization V, are sequential in nature. A player makes a move, then the next player makes a move, and so on. The details can vary, for example:
There could be two players, or multiple. This number is tightly bound for scaling reasons, which we'll discuss later.
The game could have perfect information, like chess, where all players see a move as soon as it is played. The game could also have imperfect information, like Civilization V, where players see part of a move, but the effets may be obscured by fog of war.
The players may play in a consistent order (chess, Civilization V), or in a somewhat random one (D&D's initiative system).
All of those things are more or less orthogonal to the turn system. Players play turns sequentially, so I'm going to call these sequential turn-based games.
Sequential turns make scaling the number of players up difficult. Even with only 8 players, any given player will spend most of their time waiting. While 8 players are a lot for most turn-based games, it's nothing compared to an MMORPG.
An alternative to sequential turn-based play is simultaneous turn-based play. In simultaneous turn-based play all players issue their moves at the same time, and all moves are played out at the same time. The simplest example is rock-paper-scissors, but Diplomacy works the same way. More recently, this system has been explored by the top-down tactical game Frozen Synapse.
While simultaneous turn-based play gets us closer to making massively multiplayer turn-based games feasible by turning a linear scaling problem into a constant time one, we're not quite out of the woods yet.
Consider what happens when a player does not make a move. There are a few reasons that might happen:
The player is not playing the game right now.
The player has stopped playing the game altogether.
The player may be in a hopeless position, where stalling is better than losing. (Stalling may tie up lots of enemy resources.)
If you've ever gotten frustrated at a multiplayer game that has a "ready" system before you begin a game, but had to wait because one of the players disappeared; this is essentially the problem turn-based games face every turn.
There are a number of ways to mitigate this problem. Games can duplicate playing fields. That works for both sequential games like Hero Academy and simultaneous ones like Frozen Synapse. If a player doesn't make a move, that particular instance of the game world doesn't go anywhere; but you can play any number of games simultaneously.
For this strategy to work, the playing fields have to be independent. You don't lose heroes or soldiers because they're stuck on some stale game. The worst possible outcome is that your game statistics don't reflect reality.
That works, but rules out a permanent game world with shared resources. If there's a larger story being told, you would want these worlds to be linked somehow: be it through shared resources, or because they're literally the same game world.
There's a number of creative ways to get out from under that problem, usually by involving wall-clock time. For example, if a player doesn't respond within a fixed amount of time, they may forfeit their turn. Fuel consumption might be based on wall-clock time, not turns. 1 There's a lot of degrees of freedom here. Do you use a global clock, or one local to a particular area?
A global clock is probably simpler, but poses some game play challenges. How long is the tick? Too fast, and a player may see their empire annihilated while they're sleeping. Too slow, and the most trivial action takes forever. There isn't necessarily one right answer, either. In an all-out cataclysmic struggle between two superpowers, a complete tactical battle plan may take a long time. Any timescale that isn't frustratingly short for that situation will be frustratingly long for anyone trying to guide their spaceship (or kodo, depending which universe you're in) across the Barrens.
Local clocks have their own share of difficulties. You still need to answer what happens for anything that isn't in a particular battle; you still need to answer what happens when battles merge or diverge.
I'm currently exploring the shared global clock. In order to mitigate the issues I described, I'm contemplating two ideas:
Allow programmable units; a la Screeps, CodeWars...
Allow players to plan several turns ahead of time.
These are, of course, not mutually exclusive.
I don't particularly like this, because it "breaks the fourth wall" in a sense. If my engines are still consuming fuel real time, why can't the enemy fire missiles? Either time is stopped, or it isn't. Sure, games can be abstract, but that feels like an undue inconsistency.