The first thing to understand is that the article would only would classify something as a milestone if it was truly a first. If something happens for the first time since 1912, it doesn't count. If you pitch 2 consecutive no-hitters, that doesn't count either, because it's been done. You would need to pitch 3 no-hitters for it to be considered a milestone.
If you're willing to stretch it, which I guess they're not, there will always be firsts because there are so many different stats in baseball (such as the first time a left-handed closer has saved 30 games at night with a SO/BB over 5.00).
Luckily, there are plenty of rare events (and even firsts) that could happen at some point in the near future. Even if it's the first time that a baseball event has happened in someone's lifetime, that will probably be good enough. Here are a few:
- 7 hits in a 9 inning game
- This only happened once in the 20th century: Rennie Stennett on September 16, 1975 for the Pirates in a 22-0 blowout at Wrigley
- Stennett was the last player to have 7 hits in a game, even including extra inning games (it happened 3 times in extra innings: 1932 (Johnny Burnett), 1962 (Rocky Colavito) and 1970 (Cesar Gutierrez)
- There have been 38 games where a player got 6 hits since 1976 (only Kirby Puckett did it twice)
- Single-Season Record for doubles: 67 (or even 60 doubles)
- Earl Webb set the record in 1931 with 67 doubles. The closest anyone has gotten since 1936 was Todd Helton, with 59 in 2000. Even if is 67 is out of reach, 60 would be a great milestone to reach.
- 100 stolen bases
- The modern record is 130 by Rickey Henderson in 1982. The last player to have 100 was Vince Coleman in 1987.
- No one has ever reached 80 since Vince Coleman in 1988. The most stolen bases in a season since 1988 was Jose Reyes, with 78 in 2007.
- This is the type of thing that doesn't seem very plausible now, but will probably happen at some point. For instance, no one stole even 80 bases between 1916 and 1961, but Maury Wills stole 104 in 1962.
- 2 perfect games in ML in one season
- Several times, there have almost been 2 perfect games in a calender year (Jim Bunning in June 1964, Sandy Koufax in September 1965), (David Wells in May 1998, David Cone in July 1999), but not since 1880 have there been 2 in one year (they were actually 5 days apart!)
- Hit a Home Run in 8 or more games
- The record is 8, held by Dale Long (1956), Don Mattingly (1987) and Ken Griffey Jr. (1993)
- Kevin Mench had home runs in 7 consecutive games in 2007, as did Barry Bonds in 2004 and Jim Thome in 2002. Right now, Chase Utley and Matt Kemp are at 4.
- This would be one of the best, because everyone could follow it over a week or more, unlike a one game event like 7 hits
- The single-season record for HR by a 2nd basemen
- The current record is 43 by Davey Johnson in 1973. Chase Utley already has 6 home runs this year, so this is one that could fall at some point.
Update: Ubaldo Jimenez tossed the first no-hitter in Rockies history tonight, in their 18th season. The Mets (49th season), Padres (42nd season) and Rays (13th season) still have never had a no-hitter. The only no-hitter tossed in Colorado (Mile High or Coors) was by Hideo Nomo for the Dodgers in 1996. If Jimenez tosses another no-no in his next start, he'll tie Johnny Vander Meer and have the first home no-hitter in team history.
Update: The Mets-Cardinals 20 inning game tonight was the 35th game that went at least 20 innings since 1920, but the first with over 26 combined strikeouts. The Mets and Cardinals combined to strike out 35 times: Mets' hitters struck out 16 times and the Cardinals had 19 strikeouts.