I would say start small and build from there. I have been working on this whole universe since at least OOTP 16. I just keep adding to it as I go. As you will see, my uniforms and logos get better over time. At first I was using a lot of the great stuff I found here on the forums. TXRanger has a bunch of great stuff. I use an Excel sheet to plan out every five years or so. This includes the current iteration and then any changes: expansion, relocations, new leagues are color coded so I know what to add when. It's been fun to make new team names, logos, uniforms and such, but I'm also a Photoshop nerd.
Here is my planning spreadsheet:
Baseball World.xlsx
Cuban League in 1900 with 6 teams
expands in 1915 to 8 teams
expands in 1950 to 10 teams
expands in 1962 to 11 teams
expands in 1970 to 12 teams
English Baseball League in 1906 with 8 teams
expands in 1915 to 10 teams
moves to a relegation system in 1917 with current members founding the English Champions League
becomes Tier 2 of English Baseball Association (top 2 promoted, bottom 2 relegated)
expands in 1917 to 10 teams
expands in 1925 to 16 teams
Canadian Baseball League in 1910 with 10 teams
expands in 1916 to 16 teams
retracts in 1938 to 14 teams
retracts in 1955 to 12 teams
English Champions League in 1917 with 10 teams from the English Baseball League
becomes Tier 1 of English Baseball Association (bottom 2 relegated)
English National Baseball League in 1925 with 22 teams
becomes Tier 3 of English Baseball Association (top 2 promoted)
Japan Professional League in 1935 with 8 teams
expands in 1963 to 10 teams
Mexican National Baseball League in 1935 with 8 teams
Australian Baseball League in 1965 with 8 teams
Caribbean Professional League of Baseball in 1974 with 8 teams
Latin American Baseball Federation in 1988 with 12 teams
Korean Professional Baseball in 1995 with 8 teams
European Baseball Federation in 2000 with 16 teams