On my server there's a big shortage of Primal Air. I've tried farming it in the past using supposedly the best spot(s), but it was honestly horrible. After looking at the Wowhead for Primal Air I noticed there's actually a transmute recipe for Primal Water to Air. I will say as a warning that it apparently does have a 1 day CD like a lot of transmutes, so for some people it might not be worth the time. But at least on my server, there's not 1 Primal Air on the AH and I know that it's a constant shortage, so at the moment I could make a lot of profit just doing my daily CD for this after getting the recipe. To get the recipe you do have to be revered with the Cenarion Expedition from TBC, and there's guides on Wowhead for grinding out that rep as well. I'd recommend checking the stock of Primal Air on The Undermine Journal for your server, and if it's relatively low and you have at least 1 alchemist, go for it. It could be a decent way of making a little more passive gold, just transmuting one Primal to another. I'm probably actually gonna start rolling another alchemist at some point, and this discovery has definitely prodded me in that direction as doing this transmute won't interfere with other TBC transmutes, because there aren't default TBC transmutes as far as I'm aware.

I personally wouldn't recommend Bloodmyst Isle. There's a ton of quests but a lot of those quests have absolutely horrible drop rates on their items. A lot of the quest mobs also have very slow and/or spread out spawns so if there's other people there you're gonna be waiting for respawns or spending a lot of time traveling between spawn areas quite a bit. Overall it's just an incredibly slow zone as far as EXP goes.
From “The Burning Crusade” expansion, weapons like Klinge der Zauberei, Die Nachtklinge and recipes for different weapons are really valuable. The best example is Pläne: Teufelsstahlangschwert, worth 500.000+ Gold. These items are World Drops, you can find them in all raids and dungeons. I don’t know any Open World Drops, but that does not mean there are none. Another great transmog source are world bosses and rare spawns on Outland.
Anyway, however you choose to get to the addon's menu, choose guide list and then pick the zone you want to quest in. There are some zones that will autoselect based on quests in your log, but not all guides or all quests are set up that way. (If you don't want to switch to a guide that auto-pops up choose "not now" or "never") As you can see in the screenshot, there are various categories of guides. Most max level content (such as Argus) is found in Achievements rather than leveling.
First of all, the value of transmog is decided by rarity and how they look. Some items like Minenarbeithut der Tiefen or Pendel der Verdammnis (they can drop in Uldaman) are worth millions of gold. They are worth so much, because they have a really small chance to drop. Other items are bought well because they look good. Best example in this case is the Glorious set, its legplates Glorreichen Beinplatte reach a 100.000 Gold value easily.