[PTTG]

How about a worldline where middle-ages Europe had different marriage rules that favored consolidation of states, eventually leading to a mostly centralized power system. This whole arrangement turns inward and decadent, leading to a stagnant, isolationist period for centuries.

The value here is that the cultures of Turtle Island are untouched by the Europeans, and the practices that were lost are still around, let alone the languages and technologies.

Homeline scholars would find it fascinating, and First Nations from Homeline would have an unusual opportunity to recover a lot of lost history.

Fun complications would include assembling a team to do subtle anthropology without interfering. Not to avoid shifting the timeline, but rather to protect the anthropological value of the people.