Like I implied in the other Master of Magic thread, a sandbox experience does not means that you have no objectives at all.
I could image a MoM that could work with or without AI players, with the main objective being you against the map, playing until you fulfill one of several winning condition.
Exemple of possible objectives:
- cast the spell of Mastery
- finish some sort of utimate quest
- survive until turn XXX
- if you add AI players you can then add the usual domination victory where you win if you are the last survivor, or maybe a diplomatic victory if you canbuild a strong alliance with all surviving factions.
If you want to press the player to act to prevent excessive turtling, there are several solutions :
- AI players coming against you could wel be replaced by monsters and minor factions, with dungeons, nodes and other special location acting as spawn points for appropriate monsters or armies, giving another reason to clear them and a source of constant ennemis as you expend.
- like suggested by GfireflyE, random or semi-random events when scertain conditions are met could also be a way to do it.
You ignored a certain quest to destroy an evil cult for too long ? Too bad now they have summoned a major demon and are gaining stregnth. Their lair has now gone from a low level one that a small army or hero could clear to something that will be much harder to do, and it is spawning minor daemons and fanatics stacks harder to ignore.
You didn't kill the dragon whelp when you got the chance ? Now he has grown bigger and started devouring your peoples and asking tributes or else ...
The bandic camp that you ignored near your frontier has now grown into a small independant citie over the years, and is blocking your access to local ressources and land.