Vital:
- NFL rules
- Up to 32 teams
- A good Franchise simulation engine (i.e there shouldn't be 4 RBs over 2k yards)
- A robust player / team editor
- Forza / BB logo editor
Would be nice:
- College rules option (including 4 years of eligibility for Dynasty mode, etc.)
- Ability to include more than one league in a franchise mode (a-la OOTP)
- Up to 120+ teams per league
- Editable playbooks
- A rich fictional default league (again, like the ABA you get with OOTP)
Unlikely to be possible, but worth mentioning:
- Arena football and CFL Rules
- Custom rules
- A BB style physics engine, meshed with a 2k style animation system
- Fight Night style face editor
- A play editor like Maximum Football.