I have numerous milestone collections including 500 Homerun Club, 3,000 Hit Club, 300 Win Club, and 3,000 Strikout Club. They include Topps cards from the year a player entered the club. For example, Ken Griffey Jr. hit his 500th homerun on June 20, 2004 so his 2004 Topps is in my collection.
Another favorite subset of mine is the Topps All-Star Rookie Team cards. Although I have quite a few of the cards through the years there are much more that I need.
My plan is to add award winners to my mini collections in the near future such as Most Valuable Player, Cy Young and possibly Rookie of the Year.