You should get more drops and xp for each player in the game. This 3,5,7 dance is old and not very fun. I play with my wife a lot and it is sad that players 2 does nothing for the drops.
I guess in a couple of years when he is older i can hopefully get my son to play for some /players 3 bonus 
Two players in the same area: p2/2 have the same drop chances as as p3/1.
Player command should not be a thing at all
But instead be replaced with a effort driven monster empowerment
That way people invest time and effort to get a challenging yet rewarding content
Once you kill baal in hell, the mechanic unlocks
I have 5 concept for such mechanic
demonic list: where each time you kill a super Unique in a game, the difficulty cranks up
the restless darkness: where full clearing an area makes monsters respawn like 5-10min later but harder and more density
king of Hells: where your objective is full clear the game, the more monsters you kill the harder are the remaining ones
the 9 circles of hell: where completing hell unlocks the next level of hell and so on
Natalya bounties: where she contracts you to clear specific areas where monsters are gathered to prepare to assault a town
This would be 1000x more interesting as a gameplay than just boringly typing /player 7
Well i kind of meant the players count online and not singleplayer /players 
But according to the guy above being 2 on the server should increase drops, although i have heard only odd numbers affect drops and even numbers affect experience