@LeeC2202 OH, thanks a lot!
It may not be exactly the way I imagined but probably no one will take on this suggestion when there is a close alternative.
I'm sorry if this post was unnecessary: I couldn't find these mods.

I think it would be awesome to have a special action/move for grabbing pedestrians (additionally using them as a shield) and then possibility to execute the victims or push them away. (Kinda like in the Saint's Row 2, but without throwing people so much)
I'd also like a separate key for pushing people - apart from the existing melee attacks. It would be fun to push them around and throw them into the water and so on.

I'm surprised that the NPC's in GTA5 are often so meek. You can ram their car and all they're going to do is to flip you off like "oh, i sure showed you for wrecking my half a million dollar car - that'll teach you".
So I'd like to see more people getting out of their vehicles, trying to fight. Also more pedestrians doing so when you push them or bug them.
Vital element of that mod would be making gang members spawn more frequently or giving guns or melee weapons to more people, including pedestrians.
When it comes to details, I think these rates should be about right:
60% of people inclined towards a fight when aggravated;
40% of people with melee weapons;
20% with guns.