Its not possible to take you serious with a response like this and I do not mean that to come off in an offensive way. You cannot eliminate boosting without changing some of the core concepts of the game.
The easiest way to get levels will be full parties in boss runs. To fiddle with that core concept in any meaningful way, would be changing the entire game. Few players “Farm Gear” by running boss runs in public games.
The way it works is someone who can clear makes a game because they are bored or need xp. The people most eager to join these games are usually the lowest levels who cannot contribute. It is typically harder for these guys to loot because they will die, but if they do start picking off loot like crazy the guy clearing will say something. Usually players just ask for drops they REALLY need and I have seen a lot of people give it if its not something they want or need.
If someone(s) also geared for the content joins the game and is helping clearing most times there’s no issue with that guy also looting. I have seen more arrogant people tell that person to make their own game. Most times it just makes the runs go faster.