My answers: 1-2-1-1
And the ideas:
If it is to be a like with 4 quest, I demand moar!
If those quests will be related with the answers above 1 by 1, here is the ideas (They are based on my options and the scenarios are created with a merge of my answers so 1st part is related with causing a problem, but includes stuff about hunger to the power):
Background tale: We meet a warlock after some quest chains that leads us to a secret chamber which has the thinnest point where nether and the earth realms are seperated. However when we get there, we meet another warlock there ambushing us slyly after a conversation. After that dude is beaten(Should have many mechanics that the way to kill him should contain pet twisting (A cast that has to be silenced and silenced only, a stun, a knockback etc) and should not be trivial by any means. After defeating the warlock, he tells us the story:
1. Causing a mess and be forced to fix it: The dude tells you how to obtain extreme power so that you can rule every demon, get extreme fel power, however the dude couldnt do this as he didnt have the potencial and a warlock who is stronger than him already tried obtaining the power and get devoured by magic and became a part of the nether. However since we defeated the dude, he lets us thru. We try to open the nether portal and let the fel magic leak towards the world. However we messed up and demons started to come to our realm. Lets say there is 10 demons with different abilities (Numbers can be changed) and we have to switch between all of them with enslaving. Killing one immediately makes us defenseless to another ability so we have to kill all of them at once. The quest criteria is to kill all of them after enslaving all of them. Once we are done, we can use our powers to small the portal down.
2. Learning about where demons come from (And also the limitations): We use our eye of kilrogg thru the gap as it is small enough to get in, but while the gap is small enough that it doesnt let any bigger pet to get thru. We peek the nether and learn those stuff. I dont have any ideas about that, however a minigame like Leaping Hatchling quest would fit perfectly. Finally when we learn about the stuff, we can put a demonic circle inside with the eye of kilrogg and make our way inside.
3. Learning where our powers come from: There is a well and blabla, it is storymakers job. However the demons inside cannot get so close to it, while trying to stay close (It is like fire IRL, you wanna get warm, however too much warmth is burning). The rest is some stories that can be learned with someone elses writings written in demonic and causes flashbacks like Gul'dan's writings in WC3: The Frozen Throne.
4. Puzzles: What comes in my mind is: There is a flow of fel magic well towards outside and we can basicly devour-change the way-block that fel magic flow. Devouring a lot makes us unstable and die eventually, change the way causes some damage on us, blocking requires a pet sacrificed or usage of fel magic. This way, since there will be moving particles here, things will be rng, a single fixed path cannot be chosen. We get as close as we can and when we get to the part where we cannot go thru(The intended finish), our body sparkles with fel magic, green stuff leak over our body. We learn that hunger has a limit and should not cause our own destruction as our body is already infected (But still enough for us to live with), we are ok with what we got. We become wiser. Open the gate back will our new powers (Since it was a one way ticket before since we went in with pure hunger for magic, without thinking about consequences, we can think of that part as we are inside) get out, seal the portal, maybe smash the place with our new powers since it can bring us new people to compete with us, and can bring destruction to Azeroth.
Its just written, didnt even think about it and I think it is a good one. And I kept it old school since Pandaren has no stuff with fel magic.
Also another idea for the green fire: It would be awesome if we can get a toggle on-off spell which is completely visual, and makes us burn in green fire, or leak magic from our body, similar to some oozes. I always found disgusting oozeling as an awesome pet and this can bring some more stuff that way, a.k.a charisma while not casting stuff.
Thank you for reading. Nao.
Edit: Paragraphs to make it more readable.
Notes after reading other people's ideas:
1. Item normalising should definitely be the case. I dont want it to be trivial. Superhard stuff mewantz
2. The place can be in: Ahnqiraj, BT(Can also relate the stuff with what Illidan did with Skull of Gul'dan).
3. My puzzle part can include much more steps and mine should be the last part if it should be a part since we are walking to the core of fel magic.
4. Maybe solo Illidan(Dunno if its possible on LV90, but if its not, while having the quest, Illidan could be phased and have less hp or damage to make it killable. Also it brings a story flavor as he is one of the biggest power hungerers.