I've been thinking about MAD in a fantasy setting.
Plenty of fantasy worlds have world ending threats, and many more have spells and artefacts that could be if combined in terrible, brilliant ways.
For MAD to work it does have to be mutual. There needs to be time for the other empires to react and there needs to be no cheap counter to your offense. Asimov wrote plenty of stories about how MAD broke down one forcefields were invented.
So each player names their city state or larger empire, and the magic/artefact/weirdling they hold that makes attacking them untenable. If you are playing in the current STGODS game (or would like to) even better

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Hyenorks. Hyenorks get MAD. They live in a desolate artic tundra, the kind with a winter that chews up your army and swallows bits whole. They sleep in snowdrifts and shrug off small wounds. An invasion of their land is incredibly costly and temporary at best. Winter is always coming, and when snow lies deep around your own keep in your soft southern lands, then comes a hungry enemy that can move freely through the storms.
It is said invading Hyenork lands costs three times. Once for the invading army, the second for the rescue force, the third for the subsequent raiding and consumption of the farming population and their animals.