It wasn't until 54 years into the expansion era that we had a World Series matchup between two expansion teams. (Mets vs. Royals, 2015) It's now happened three times since then: Nationals/Astros in 2019, Diamondbacks/Rangers in 2023, and now Brewers/Rays in 2026.
Significantly*, the 2015 Series is the only one of them that didn't feature at least one team that had changed its name since its inception, and in two of the three, both teams have changed their names.
2019 was the Expos vs. the Colt .45s, 2023 was the Diamondbacks vs. the Senators, and this year, it's the Pilots vs. the Devil Rays.
*not really