When you have finished all your quests which can bring you to the 20th rating you will come to the Redridge Mountains, Duskwood, Wetlands and the Valley of Ashes which are the most possible places when you are in 20th rating to 30th rating.
The Valley of Ashes is beyond my choice and recommendation. The reason is that the map of the Valley of Ashes is too expanding and the quests route too prolonged. The Redridge Mountains rates between 16th to 26th, Duskwood 20th to 32nd and the wetlands 20th to 30th.
When you are in the 20th rating, you are supposed to go to the Redridge Mountains and finished all the quests below 20th rating. Urkrass are mostly lying in the lower places. The Urkrass-centralized places I referring to can be your best choice for your raid. And then you can go to Duskwood and finish all your following quests and finally the remaining quests in the Redridge Mountains. You are now in the 26th rating.
In the way to the wetlands you should firstly finish the quest from Robert Swain in the Duskwood. Since you are now 26 you will enjoy a more quick speed in the wetland. There you can put the quests of Max power aside. Enjoy yourself. You have been in the 28. And go back to the Duskwood to pass on your fulfillment of quests, the legendary reward of ring from Stavanger as a surprise. The wetlands boast the great number of quests. Go ahead and speed up to 30.
The places for 30 to 39 can be listed as Hillsbrad Foothills, Arathi Highlands, Alterac Mountains, the Shimmering Flats, the Swamp of Sorrows, Locations in, Dustwallow Marsh, Badlands, and Hinterlands etc.
 What I recommend to head on are the places like Hillsbrad Foothills, Arathi Highlands, Alterac Mountains, Dustwallow Marsh, Badlands, and Hinterlands etc. I will share the reasons with you. The way to the shimmering flats is too far to get in time and the quests for a small amount. The swamp of sorrows has little quests neither. Meanwhile, the Locations in map is too expanded for you void of any flights and rides. It may be a too long journey. Finally the Badlands is not worthy your hard works.
When you finished the quests handed from Max power and go up to 30 you can check in the Arathi Highlands. Remember to jump down when you come across the Gennaro Sardo ridge. Find the suppressed dwarf and get the letter in his hand. When you finally come to the asylum valley, you should not stop even a minute, in the other hand; you must go straightly to the Hillsbrad Foothills. In fact, the possible quests before 35 can be counted down. Having finished the quests of Hillsbrad Foothills, Arathi Highlands and other places you can go up to 38 or 39. In the way, remember to do accept the quests along the coast. Now you can head to Dustwallow Marsh.

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