Suggestion
For the long time fictional players, a "bloodline" feature. Basically, multiple generations of players from the same family, like the Bonds or Griffeys, or relatives from the same generation, like the Robinsons or Ripkens (ok, so Billy was a pro on a technicality). A notation in the history or on the players web page about "being the son of so-and-so", or "so-and-so is his brother and also a major leaguer"
Reason
One of those things that make baseball, and all of sports, a bit more interesting, being able to link the past to the present and future.
Priority
Highest of the high?
Kidding, obviously very low, easily more cosmetic than anything substantive