I've been fictional-only since I started playing OOTP. Used to be that I would start a dozen different universes, play them for a little while, get bored and create new ones. I would GM a team here and there, but mostly I was into it for the world building. I may go back to that one day, but...
For 16, I decided to try something different. I created a very small universe- one ML with 16 teams and simmed 10 years. At year 10, I created a player and entered him into the ammy draft. I started a dynasty report (since abandonned, sigh) where I was that player. I followed him very closely through the minors and eventually to the majors, writing dynasty reports from his perspective. In this way, I got to know my league inside and out while it continued to build history. Once my guy retired, I created a human manager with his name, and started managing my way up from the minors.
I was lucky in that it only took me three seasons to get a big league GM job - with one of the worst teams and worst owners I have seen in 11 versions of this game. I am now in season 32 of the league, and almost all of the current coaches in the league are guys that "I" played with or against. I am so immersed in my league that I don't want to take the time away from the game to update my dynasty.
It's a lot of work to create that level of immersion, especially when you consider that you can get that out of the box if you play MLB quickstart. But I love creating the universe that I play in and not having external preconceptions about which players are destined to do what. Besides, I refuse to live in a universe where Portland does not have a ML team.