Off the top of my head, I would say that these are a must:
- Ability to save and load custom rosters, as well as share them
- Ability to reorder lineups, reactivating injured players in the official roster updates if desired
- Ability to trade players between teams, release them into the free agent pool, and sign free agents
- Ability to edit player bio data and ratings. This includes all ratings/tendencies, jersey numbers, positions, contract information, etc
- Ability to create new players, and fill in detailed bio data/ratings (ie give them full Draft information, all the same ratings/tendencies as original players, etc)
These are things that I'd also like to see, but aren't as pressing, because we can still make good custom rosters without them.
- Ability to use face scanning for created players, not just your Rising Star/LIVE Pro-Am player
- Ability to create custom teams, with a selection of pre-made images, and custom image importing
- Mass editing functionality (eg if you want to lower everyone's speed by 5, you can make a global change)
- If there are any special teams, the ability to edit them as well