Things that proficiency bonus increases:

 1. Saves a character is proficient in
 2. Attacks with weapons a character is proficient in
 3. Ability checks made with skills or tools a character is proficient in
 4. Bards and Fighters have ways of adding half their proficiency bonus to untrained ability checks
 5. Bards and rogues (and some feats) can add double proficiency bonus to some checks

So, how can we simulate this with existing magic items?

 - A Stone of Good Luck (uncommon, attunement) gives +1 to all ability checks and saving throws - better than a +1 to points 1, 3 and 4
 - A +1 Weapon (uncommon) gives +1 to attack rolls with it - worse than point 2 in general, but for a character who only uses 1 weapon not much of a difference

An item that gave +1 to proficiency bonus would probably have to be at least Rare, given that it combines the properties of two uncommon items (and allows for the use of a separate weapon), but I don't see it breaking anything. In a few cases it will add two points to a skill; this is nothing to be sniffed at, but if the item you're crafting is meant to be Legendary, you're still well within bounds of that. (I'd probably call it Rare, but an argument could be made for Very Rare, since +2 is a reasonable boost.)