Six Days in Fallujah

Others have recommended these things in the past and I wanted to provide my take on it.

For taking damage, I think small objects like shrapnel should damage players in order to add in more immersion. Additionally, if the player or teammates are hit at certain parts of their body, this should be affect their mobility and ability (e.g., bullet hits leg results into limping, bullet hits arm affects the ability to aim, etc.).
I think SWAT 4 had a similar mechanic where if one was shot in the leg, the player would be limping. However, the issue was that the player couldn't be patched upon taking their injury.

This could be remedied by using medical kits and tourniquets in order to patch one's self or your teammates. Likewise, teammates can patch each other up dynamically when one of them is hit.

If implementing this mechanic for teammates proves to be a hassle, I think putting it only on players would also be viable.

Anything but health regeneration would be good for the game. I think it needs to be somewhere in the middle, not too complex, but also not too simple.

A bit like Escape from Tarkov. Maybe that's too complex, but it would be cool.

I take it none of you have served or saw combat? Maybe do some research what happens in real life when someone gets shot or sharpnel. Here is a hint, you do not just put a "bandaid" on it and still do your mission. That's not how the real world works.

So if they want to make it a documentary video game, they should get this right or it is just another thing they get wrong.