Adding New Proficiencies (DMG)

The proficiency lists in the Player's Handbook are extensive, but not comprehensive. The proficiencies given are the ones that characters will most commonly want or need, and those that have significant, specialized effects worthy of explanation. DMs and players will certainly think of proficiencies they'd like to add.
Wherever the idea for a new proficiency comes from, the DM is the person who decides whether to include it in the game and what its effects are. This is not a decision for the players, although they can offer suggestions and advice. Only after a new proficiency is approved by the DM can it be used in play.
One important factor to remember is that no proficiency should be beyond the science and technology of the age. There's no proficiency on the list that allows a character to build a gasoline engine, and with good reason. A gasoline engine is far beyond the pseudo-medieval society presented in the AD&D game.
At the same time, this is a fantasy game filled with magical effects and strange powers. With magic, it is not impossible to have outlandish and amazing proficiencies if players and DMs want them. They may have a serious effect on the game, however, and must be carefully considered.
The majority of new proficiencies are going to be those related to trades. Most of these have a very minor game effect, if any at all. They give the character specialized knowledge, but it is up to the player to make some use of it.
A character with the skills of a glazier (glass-maker) does not gain a great advantage. Although, if necessary, he could support himself by making small glass vials and other items for local mages and adventurers. Still, there might come a day when knowledge of glass and glass-making becomes vital to the success of an adventure. A clever player is always looking for a way to turn knowledge to his advantage.
When a player proposes a new proficiency, have him prepare a description of what the proficiency entails and allows. Then consider what the character could gain from it. This is not to say that the player is trying to pull a fast one (some will, but give them the benefit of the doubt). Instead, it is useful to imagine ways the proficiency could be abused. If something horrible or game-busting comes to mind, fix it. Never allow a proficiency into the game if it seems too powerful.
Make whatever changes are necessary in the description and then offer it to the player. If he still likes it (after all the secret powers are stripped out), introduce it into the game and have fun. Sometimes the only thing that can be kept is the name of the proficiency. Don't be distressed by this. Most players will be satisfied with DM changes, content simply to contribute something to the game.