Has there ever been an expansion team to make the playoffs in its first season, much less compete for the championship? In any sport?
No, I think the closest was when the Colorado Rockies made the playoffs in their third season in Major League Baseball.
(Except, of course, in the first year of a league, when they’re all first year teams.)
Las Vegas got special advantages this year in the way the draft was set up, because the NHL didn’t want yet another new team that gets mired in year-after-year of losing.
Seattle will probably get the same draft advantages when they get their NHL team.
I think the Arizona Diamondbacks made the playoffs in their first season of major league baseball, but didn’t win the world series until 2001.
I think the Carolina Panthers and Jacksonville Jaguars both made it to the conference championship in their second seasons (same season).