They would need to (at least):
- move with the team
- be able to use cover in a meaningful way
- not block your way
- shoot at components in a balanced fashion (i.e.: not have perfect accuracy, but not have terrible accuracy either)
For PvE, the AI can be rudimental (devs can play with movement paths, spawn points and ranges to give you the illusion of intelligence), and it would still be fun.
But for PvP, i'd rather have a bad team-mate or wait a bit more.