Well, we have fortress mode, we have adventurer mode, someone was going to suggest the aptly named god mode sooner or later
xD. I've seen a certain clamouring now that we're nearing the army arc for starting off at that level, rather then the current outpost beginnings. On the other hand, when the army arc comes about, at certain levels of power it will be almost inevitable that your going to have some dwarfs squalled away somewhere, though of course i expect there to be proportionally huge threats. Thus, an idea. Coming into being at a time of your choice, whether at the start of the world or in the middle of it, you are a god, initially at least of the dwarfs. Your goal is a blend between that of fortress mode and adventurer mode, except that you are now attempting to ensure your own survival and worship at the same time as building up, securing, sciencing and possibly looking after your worshipers. What makes it interesting is that you have competition, some weaker and some far more powerful, some who may even have created you or be part of your own nastily divine family, as well as the little fact that your control is limited, depending on how venerated and free-spirited your worshipers are and the might you have attained. You'd be able to shape the world if you were strong enough, or become a weak and fragile avatar on the lower rungs, perhaps even simply a ghost hoping for a open-minded or easily dominated soul to chance on one of your forgotten temples. You'd balance the relationships of your followers, their alliances and your fellow gods, fell creatures and artifacts, all the while trying to master the element you've ended up with it and use it to your advantage.
It would also be one of the potential, (as it would not necessarily happen, but be one of many possible occurrences) hopefully multiple ways fortress mode would become ever more of a challenge. One day, when your dwarves in their foolishness begin to take elation in their success and become thankful for it, you will be born, shaped by their past and their hopes and dreams, being born to one of the other gods, or simply attracted to a people that has grown great from elsewhere. This would, of course, not be restricted to empires alone, as even a continually successful fortress may create you in their foolishness, and though somewhat dictated by the nature of your followers and the scale of your success, would also have a hefty degree of randomization, drawing from your origins, parentage, regions, enemy's and failings, so even a utopia might have a god who must rely on his wits and not his brawn or a small gathering of settlements a god strongest when manifested in the mortal realm.
Which brings me to my next point; you and your powers will be called on often, and whether you leave your pathetic followers to their leavings and conserve your strength or attempt to right every wrong will be a pressing concern. The most pressing of cases may even call for an avatar, or indeed your personal manifestation if the need calls for it. With this in mind, being a demigod in adventure mode would really mean something, and just as much for a god themselves. You might be trapped in this form, or get inventive with your approach, or even scare away the worshipers who cannot believe in your physical presence, or worse, your imperfections from what they were led to believe by their priests. The interaction with other deitys would be the really interesting part to me however, be it through cosmic games of chess, house pollitics or a war to shake time and space itself.
I could ramble on and on about this, but the point is introducing another level of potential challenge, (as i should restate that this is far from a guaranteed outcome, merely one of many the game could throw at you), as well as starting point, to long term fortress mode, amongst others of equal grandeur (such as refusing to manifest a god and having the magical and angry deity fallout, or otherworldly visitors, or a reshaping of the world, or beginning to lose control of your dwarves, or other game changers). Your thoughts are more then welcome.