@TankHunter678 I would really shy away from promoting the solo "one man can do everything" aproach. I would rather see groups forced to specialize in order to bring a mix into a fight. Allowing a single player to bring everything needed to a fight is bad for team play. Remove that and you are forced to either increase the number of mobs or scale up their HP/Damage to compensate for a full team all having the weapons best capable for the encounter.
EDIT: What I mean is, I think we should only have a single main weapon attack, a much weaker alternate fire, a backup pistol if our main weapon runs out of ammo and a melee weapon for close in fighting.
My honest opinion is what you are saying would kill off a lot of weapon customization. People would just look for weapons that do everything enough to get through things and then not bother.
After all if they only got 1 actually useful weapon they may as well make it counter everything (to some degree). Or we wind up with mob design built around that. Then we get back to the problem you just mentioned. They have to increase the number of mobs and/or scale up their HP/damage to compensate for a full team all having the weapons best capable for any encounter imaginable.
Instead of you know, having a wide range of mobs that force some preparation and flexibility with strong defenses against one thing or another.
The problem that you are ultimately most concerned about is team play. Majorly limiting weapons wont promote more team play. What it does promote is meta weapons that do everything by themselves. As well as promote simplified enemy design.
Mob design, and encounter design, should naturally encourage more team play. If a person is not prepared for a mob type there should be some weakness that can be exploited, but is vastly easier to exploit with the help of another person.
Also, even in MMOs you got a large group of people who wont have others around at all times to help them. They need to have some flexibility to be able to do some content on their own. If its one of those games where you absolutely have to have other people around to help you a massive chunk of potential player base is instantly lost. They wont try it because they wont have anyone to play it with them and help them. They wont try it because they have been burned by so many other games that do that and expect another troll infested wasteland where people do everything they can to get in each others way for fun.
I promote a flexible system that should reward people for being prepared. Regardless if they are trying to do content by themselves, or with a group. They should succeed or fail based on their skills, and preparation is one of those skills.