There's probably be a way to create a simulator mission and/or a character creation tool, but it'd unlikely be in-game as that'd involve looking at the exe's source which is more or less impossible.
-Not looked into modifying simulator missions but surely they'd hold information on stuff like bad guys, objectives, level etc, so basically store all of these within a data base and create a program that writes to a text/xml file in the appropriate format (doable in C++/c# but probably time consuming) then just use nba2kstuff's compiller to compile the mission file (or something along those lines...)
For character creation it would be a lot trickier as a totaly custom character would be night on impossible but creating a character combined from other areas wouldn't be as tough
Basically, repeat the above but to create separate powersets/talent files along with a hero stat definition and then compile these. Again, programming would be a basic writing program from a database