@All: Hmmm… One theorhetical model could be something like this:
Crafter only has autoattack, no specialized combat skills. Could of course use grenades, bandages, healing and other potions, scrolls, etc.
Crafter characters only see the return of profession specific buffs. Herbalism grants minor healing / regeneration skill, mining adds a few hit points, skinning grants a tiny increase to damage and/or critical chance.
This way, a crafter is not a combat monster, but can solo some stuff, albeit slowly. Needing assistance could actually encourage small group / pair play. Obviously, nobody would take a crafter for world first attempts nor the absolute highest +Xs, but once people have enough gear and skill, I am sure four people could make up for the crafters’ lack of direct combat power in dungeons and in a group of 10, 20 or 25 having one or two people with less impact would not be a huge problem. And in the open world, a partial party of tank + crafter or DPS + crafter or even healer + crafter could acquire some resources with reasonable speed.
Also, being weak in combat would mean a crafter would be a bad idea for anyone thinking of botting as for example a druid would be more mobile, durable and have more hitting power. Crafters would not be entirely self-sufficient, because they would require help in harder content and would most likely be unable to solo many things that are possible to solo for other classes. So there would be significant tradeoffs. Additionally, “Need” could be disabled for crafters, so they could not ninja stuff from people who need specific armour and weapons (and/or there could be some crafter only gear that would only drop for them, perhaps even with minor relatively unique details like a minor smelting / herbing speed buff, +5 (or +10) to any one crafting skill or something like that).
Some people would never pick a crafter, while others might be attracted to that specific play style.
I am not sure how feasible the idea would be from Blizzard point of view or how popular it would be, but I do not think it is entirely impossible.