After you get a 99 in a skill, you can disable daily challenges from this certain skill. If one would happen to have all 99s, one could disable getting daily challenges in any skill but dung. This person would then get Dungeoneering challenges daily. You can also do this if you have 4 or less skills left to get 99 because you can't get a challenge in the same skill again when you already have one. I currently have 21 skills at 99. I disabled all skills but those non-99s. I currently have challenges in all of these skills which I haven't done and will never do. This leaves me with 4 permanent daily challenges and gets me the Dungeoneering challenge every day because I can't get a challenge again in the 4 skills I already have challenges in.