This I think it one of the core problems with COOP at the moment. Ill make a few points and encourage a discourse of fors and againsts.
I think at the moment the teams we have at the moment make the game too easy. Rather than making the bots more difficult I think the teams should be smaller. Having a meaningful impact in a game with 8 or more players is hard and most of the time its 2-3 players that carry the rest.
Honestly its enough that we have to compete against the bots let alone against the team to do well and get points. I think smaller teams will encourage slower more meaningful team work and a lot more fun. I know that the games iv enjoyed the most are the ones where its been me and 3 others. Nearly out of ammo, trying to make it through this wave to get to the next objective. Potentially smaller teams could mean the introduction of medics or maybe wounding mechanics.
Smaller teams would also mean team composition would be more important. Do you take the two slots it needs to call in support or do you forgo that to get more utility in the team? etc etc.
At the very least I would like to see this as an option in the setup menu. I think this would really add some depth to the game and allow teams to gel more and learn to work together.