An RvRvE game. MMO (because of course).
Basic premise is the most of the world is given over to standard RvR fighting over resources and territory. The gimmick is that there's a major AI threat that's also expanding and changing the world, by conquering nodes (including quest hubs) and growing stronger. Minor resources nodes might simply give a global health and damage increase, while major nodes might cause new types of elite and boss mobs to spawn during encounters, and losing a major city node will cause a world-event boss to spawn and attack random nodes outside of the threat's normal influence. Lost resource nodes are permanently consumed but can be reclaimed and garrisoned by NPCs to negate their benefit to the enemy.
The gimmick is that players can fail to counter the threat and lose. Then world is then reset by the in-game gods or whatever Powers That Be, somewhat like Planetside's continents. Same happens if they win.
With a functional reset built into the system, you can do a lot more than typical static worlds. A reset might radically terrain, new (lower scale) threats are present in different places, extensive changes to skill balance (to deliberately upset the meta). NPCs might change. The AI's strategic priorities and behavior changes each season (it can also have its decision tree/weighting prodded by devs during a season to help upset predictability). The AI threat itself might change, too - the game might start with only 1 antagonist (maybe a silithid/tyranid kind of bug race), but expansions could add new threats like shadow demons or ork swarms, or minor NPC races are given a divine boost and become the main antagonist. The "final" event might also change - it could be a world-scale boss raid (not only fighting the boss but other groups dealing with reinforcements), or claiming a primary node (shard of a god or such), or building up a city node to enact a world-level spell.
Reward systems are split between "world contribution" and "research contribution." World contribution is based on participating in events in the living world and contributing to the vE war effort. For example, taking over a node would be an in-world GW2 scale zone event chain. By helping to contribute resources for preparation and participating in the combat event itself, you earn World Contribution points. Once the the node is claimed (unless it is attacked later and needs to be defended), this event is not repeated in the living world, but you can instead repeat a simulation of the event (i.e. farmable instanced version), framed as helping to research the threat. Research Contribution goes toward catch-up gear, world contribution toward prestige currency when the world resets. I'd want a cap on world contribution per day/week (daily but you can accumulate up to a week's worth) and per event, but this might incentivize meta gaming (you contribute up to your cap for the event, then you leave and go to another event).
The resource node system would probably look like something out of Black Desert Online. You hire retainers to help collect, process, and move resources, and logging out in a node helps reinforce it while you are offline.
The skill system would probably be something between GW2 and Archeage - due to the skill shakeup every season, it's probably better not to lock people into specific classes with fixed skills. Maybe borrow the Path of Exile skill gem system, though this risks the same problem as POE (with major meta skills dominating representation), but this is helped by the skill changes every season.
The setting will need some kind of global communication net (not simply outsource this to external communities/organization on discord or community tools). This could be handled by magic or tech - there aren't enough tech settings but an advanced tech setting will bring questions about scaling beyond the scope of a single world; it's easier to contextualize the cycle in terms of divine/magic/fantasy.
There's several parts of the concept I have trouble with:
* What rewards players should be given when the world resets? Power increases from "prestiging" (as though it was an incremental game) are an obvious option but they risk being too powerful and either scaling too far or capping out. Maybe simple, free shop currency for cosmetic rewards would be best.
* Incentives for RvR. Is there an extra bonus for being part of the group that defeats the threat, so you're incentivized to selfishly help only yourself and your guild instead of the global war effort? Maybe the threat doesn't expose itself for a month or two, so the first month is given over to player infighting to become strong, developing grudges which might continue when the threat becomes active?
* Incentives for fighting to the bitter end during a losing scenario? Can city nodes "fling a light into the future" to help the next cycle if they gain enough resources (i.e. survive long enough)?
Finally, this is obviously a very ambitious project. How can it be scaled down? Can we get a skyrim-level game out of this? Single player offers more freedom in terms of how the world evolves and the economy works (since everything can run on NPCs instead of needing the involvement of a critical mass of cooperating players), but even Skyrim is a massive world. Can it be scaled down further to something like a mod of TOME, just to function as a proof of concept?