The new GURPS Ice Age worldbook was written for prehistoric roleplaying, and it makes a great "change of pace" campaign. It can be fun to play characters who can't count beyond 4 . . . But it can serve as a valuable sourcebook for other campaigns, too.
Post-Holocaust – Use the book as is, but the setting is really during a nuclear winter. The players may or may not realize, in the beginning, that there are modern cities beneath the ice! The more primitive races of human become devolved mutants.
Space – Use it as a source-book for a primitive planet. The PCs may be explorers, or they may be stranded there.
Fantasy – The PCs travel to a far land, populated by primitives. But some of the primitive shamans are very wise in their own special field of magic; there will be things that they can teach even the most sophisticated wizard!
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