Jord, all those 4 points you listed are valid, but almost every single one of them would be solved by having a class system in place, would it not?
1. Maybe lower the starting skill points per class, so traders couldn't get every tradeskill from the start if they chose trader class (although, let them, as they can only get so much especially if you lower attribute points from certain X+Y's) and would allow for a sort of economy/interchanging.
2. Makes sense, and I can understand that. The only problem is, again, if you're a fighter, why allow them to be a trader still, even if just one trade skill? I like this and understand this idea, just not sure if such a drastic change as your propositions is the best resolution to solve this idea.
3. Very true, and again would be fixed with classes. This is basically the same as #2, making (X+Y) warrior get (X+Y) trade skill.
4. I think this is something that's needed fixing for years now. There's just too much of a gap in certain parts and such. Mickey says he's adding more misslers and mages for NPCs, but who knows. I think XP needs tweaking (also Mick said he's done to help out leveling) and attacks and so forth. The drops need to be fixed as well, I don't think ANYONE disagrees with that (remove jewelry making/make them drops, etc.)
I think the biggest and/or best thing to do is to make certain types of characters useful. Like I said, I don't disagree that your plan would fix or solve these problems, but I think a class system would be a little more effective. You're basically wanting mages to become cooks, farmers, and alchemists. Why not allow certain traders to be that and let mages be primarily mages? This will demand more players to do a certain skill, while allow for a mage to be more supportive to a "group" of fighters. Should a resistance type defense system be put in place, similar to all other major MMORPG games for "parties" or what not, mages would be in high demand. They would then need pots (mana pots for example) to support a group of fighters trying to take on quests or higher leveled monsters then they're accustomed to.
In simple terms, class system allows more communication and more players than just allowing an offensive character to be a trader as well that you suggest.