Blue Jays slugger Jose Bautista is leading the ML in HR right now, with 33. He is on pace to hit near 50, but could easily end up leading the league with less than 45. Considering that he entered the season with 59 career home runs (and a career high of 16 in 2006) over 2000 PA, he would be the unlikeliest player to lead the majors in home runs since Cecil Fielder in 1990 (at least Adrian Beltre had hit over 20 in 2003), and that was because Fielder had been playing in Japan.
Bautista (still identified on wikipedia as a "utility player") has been traded for a player to be named later (in 2008; the player to be named later ended up being Robinzon Diaz), been drafted in a rule 5 draft (in 2003, by the Orioles), selected off waivers (2004) and been traded twice in one day (2004).
From 1955-1976, the only years where the HR leader wasn't an eventual Hall of Famer were 1961 (Roger Maris) and 1968 (Frank Howard).