I would cast Timothy Olyphant in the third season of Fallout, and I can prove my point.

(Contains spoilers!)

A possible casting of Timothy Olyphant, besides being a very good actor in my opinion, would be perfect for the third season of Fallout due to his vast history with roles of tough sheriffs and gunslingers in the western genre (like Seth Bullock and Raylan Givens), which would suit the post-apocalyptic aesthetic of Colorado.

The Colorado arc (where The Ghoul/Cooper Howard continues his search for his family) carries a strong "post-apocalyptic Western" appeal. Olyphant is a seasoned master of this style, with legendary performances in Deadwood and, of course, Justified. His presence would bring the archetype of the weary, tough gunslinger, which fits perfectly into the Old West aesthetic that the series may adopt in Colorado.

Aside from the dynamic between him and Walton Goggins from their excellent days in Justified, it brings a dramatic "weight" and a clash of ideologies that immediately recalls the iconic rivalry between Raylan Givens and Boyd Crowder in Justified, which would also serve well in the Fallout universe.

Ideas I have for who he could play:

1. An NCR Ranger created specifically for the series with a sheriff-like and authoritative style.
3. An Enclave Agent who would hunt the ghoul in Colorado.
5. And for me, the best idea I had, an autonomous sheriff from Boulder or Denver (Dogtown), and he would cross paths with Cooper.

(Anyone who's a fan of Justified and Fallout like me, I imagine they'll understand my point.)