I am designing a game based around a fairly realistic simulation of 'Age of sail'-like trading and combat, and I am currently trying to decide whether to make the game a limited on-line application, in which the game world exists only while all the players - 2 to 8 of them - are all online, or a persistent real-time mmo application, in which game events occur on a set schedule. As a game for a small number of players, uninteresting events could be time-accelerated away.
The trouble with a real-time MMO is that the world objects would be persistent, yet the players would be transient. Since in-game encounters and combat would take place on a schedule determined by the actions of all the players, not just those that happen to be online, players could easily miss an encounter or battle in which their vessel happened to be involved - or at least could be involved.
An idea I had for the MMO version was that otherwise idle time could be spent in training - which would be simulated, not merely administrative, and that active crew positions such as gunners, helmsmen, marines and officers could be filled by players, so one ship might have many players online in engagements, each contributing to the success - or otherwise - of the battle. These positions would be filled by players whose vessels were all engaged in uninteresting travel or laid up for repairs, using a matchmaking system to match the players' observed skill to that of the NPC into whose shoes they would be stepping. NPC skill progression would be based upon the actual skill improvement of the players who step into the role.
What other game mechanics could allow a game such as this to be playable as a persistent real-time mmo, especially when vessels could spend a long time in transit, doing nothing much of interest?