I am the DM of 5e, and during my campaigns I try to allow characters to get an artifact — a legendary magical item — as a sort of personal side quest to build their character. This is something that happens over the 'lifetime' of their character. I let them choose from the known ones or imagine one of their own. For custom artifacts I compare stats to known item to make sure it isn't too overpowered, i.e. going beyond even legendary.
Recently I had a player that wanted their character to have a magical item that enhanced proficiency. Because proficiency is such an integral component of the game, I am not sure how this would play out. Would +1 prof be more overpowered than even a legendary, or would it be more like +3 on a weapon? The player was aiming for +2, but I already shut this down. Would love to hear some opinions, especially if you have done this and how it worked out.
The player in question is a bard, but I also want to know as a general reference.