I currently have two guys like this in my Expansion Team.
First guy was a free agent signing just to play in AAA. He hadn't been in the majors for four years. Then at the age of 27, he won Rookie of the Year and hasn't looked back since. Perennial All-Star and a One-Time MVP winner.
The other guy is a catcher I drafted in the 28th round (30 rounds total). He was pretty much complete garbage until one development update bumped him up from a 0.5 star player to a 4 star player at the age of 20.