The original promise of nuclear power was that it would be too cheap to meter. If practical fusion does that, and I doubt it, but some unintended consequences would be:
1. Cheaper everything, especially aluminum and other electricity-intensive manufacturing
2. More traffic
3. More urban sprawl
4. Decline of the oil & gas industry
5. Decline of centralized power utilities
6. Gradual elimination of the US (and Canadian) power grid
7. Decommissioning of most fission power plants
8. Lockheed-Martin stock goes up a lot.
Well, perhaps this one isn't unintended.