Honestly the best thing I've ever done 2k related was in 2k14. I have the PC version so I got the ultimate base roster mod. The ultimate base roster mod includes every NBA season roster from 1960 till 2015. All cyberfaces and very accurate. It also includes every single draft class from 1960 till 2017 (projected prospects with cyber faces).
So what I did was start a career mode in I think 2004 (the year the bobcats were introduced since prior to that you'd have less than 30 teams and it would glitch and each year I would add a new draft class starting from 1960 all the way to 2017. So I had a year long (real year) career mode all with real players. So damn cool.
Aside from that in 2k15 I have a roster where I'll make random players younger or older. I have rosters where I took all the legends and put them all into one big fantasy draft. I found a way to change the ages of the legend players and acted out a scenario where Michael Jordan comes out of retirement and plays for the Hornets. I like going in and making someone like Hasheem Thabeet a 19 year old kid with tons of potential to rewrite history.
I currently have over 600 hours logged in this game, still always think of a new thing to try. Its just sad its not as easy to mod as previous 2ks but at least we got the next ten treatment. Currently I have mods that add tattoos to all the rookies, cyber faces, drummond has no headband and Westbrook has the mask on. But its a shame this isn't possible anymore
https://www.youtube.com/watch?v=5f-EfXu_jic
That mod was so fun