But a puzzle like this would have to push the boundaries of a class, so much that when people find out that our class has this little "thing" it could be seen as imbalanced.
But beyond that, the easiest way to create a puzzle without solving it by going to wowhead is to make it extremely complex and add multiple variables. So basically, sure you can get the "jist" of what you're to do, but the adventure getting there and the choices you make will still be on you.
I'd like to point out the legendary quest line, the dungeon created for it was challenging to do in early tier gear, and because of that, it was fun. So an element like that would be great to add to a puzzle, make mini-bosses drop certain items to be used in said puzzle and then have the puzzle itself. You really really should instance this if you're going to do it right, "world" puzzles are really no fun considering they have to be normalized for any tom, dick, or harry that runs across this (also note that I have no idea what you're talking about in regards to what we get, so toggle this as if it was based on a very powerful spell or object (think doomguard or legendary staff))
Now for the puzzle, randomization is always a great thing. Think account-bound type of puzzle. You loot a chest that gives you one out of (depending on how you guys do loot and/or quests) an infinite variation of a puzzle. The puzzle will always be in the same style, but getting to the end may be different. I'm going to give you an example of how I would set up an instance, there are five examples of puzzles in the order I would do them in. Obviously you are to take this with a grain of salt.
Puzzle A out of E is a maze puzzle. You have to roam the dungeon/area and find 5 mobs in 15 possible locations in order. The first mob you kill will be right in front of you and drop an item, said item will give you a "riddle" to the next location. Now sure, you can wowhead this, but reading the clue and adventuring is not only funner, but it would actually take longer to go to wowhead (this is puzzle A, its easy!)
Puzzle B out of E is a math/logical puzzle. You open a tile on a wall and you have to complete the pattern. Now to do this, I would start with a 125IQ challenge (so basically 2-3 minutes to figure out for those with IQs of 125 or over (i.e the top end of above average, going into gifted)) and have it drop by 10 points per every ten minutes of failures until you reach IQ95, this will allow players who are dedicated (and intelligent) to rush into it and try their hardest, without allowing them to exploit and slide through the 90IQ version. The pattern itself should be something easy to randomize (symbols in a certain order, or a slide table, or sodoku type) and when you "finish" the door or portal or w/e will unlock.
Puzzle C should be a boss fight that is literally a dps race while keeping us alive. I'm talking give us a full set of legendary warlock gear blessed (read: damned) upon us by some demon lord from some corner of the nether and let us have fun. Give us 2M chaos bolts, give us 200% haste, give us permasuper meta with a green aura, give us everything and let us go to town. It will be fun watching recount say we're doing one million dps and it will be fun avoiding one shot mechanics and potential buffs/debuffs. This is a once in a life time thing, make it cool.
Puzzle D should be a co-ordination puzzle, we're coming down from our high and are in our second wind. For this puzzle think Alysrazor and flying through rings. Have us ride the waves with our demon form or a doomguard or some type of flying demon, let us shoot something out of the sky while also maintaining our health. (So like Maly + Alysrazor). This fight is simple and cannot be wowheaded since you can't have wowhead play for you. :P
Puzzle E should be the hardest and should be a mix of all four. Let your imagination run wild, develop whatever you want, but be sure to recap on everything so far so its like we learned something. Make the puzzle FEEL similar, give us similar debuffs, similar challenges mid puzzle. I, personally, would have the place collapse or something, so you have to run out. But to open the door, gotta find three mobs, then open a puzzle door, then kill a mini-boss, then finally get on your steed and run through a collapsing floor.
Now, obviously a lot of this is more like "lets make a new video game just for warlocks" type of thing, but this is an idea that, if ever implemented, would excite me greatly. So take from this little fantasy I have whatever you can and I hope it shows you a view (albeit limited) of what a Warlock wants.