1. Introduce class specific "epic" weapons:

Now, going to use ranger as example since this is the class I play:
I go to trainer, and get the rumour of a grandmaster fletcher in Istaria. Later on, when reaching higher levels, I would get more info from my trainer as to where and when - basically giving out lore as to who the man I'll end up seeking is, as well as hints to where he is.

When finally reaching my goal, finding the fletcher, I'd need to prove my worth of his work, much like the best sword-crafters in the samurai lore would only make their swords for the very best of the samurai.
This may include killing a specific named mob (maybe one of those in the dungeons Vah has explained?), but should include more than just that - afterall, being able to kill stuff is not enough for most classes - a paladin needs to prove he really is as "lawful good" - a ranger may need to prove his affection of nature in some way - I'll leave this open for the professionals ;)

The bow itself needs to be different for everyone (this is the hard part). Maybe have 5-10 looks with 5-10 colors that is chosen randomly. The damage should be a bit higher than the best bow ingame (this is afterall a quest for people having reached max level) and have one or two unique techs, chosen randomly between, say, 20-30 (with possibility of more added). I don't want a superuberitem - I want an item just a tiny, tiny bit better than what crafters can make. fx 2-3% more damage, something in that alley.
Of course, the bow will NOT be tradeable to neither NPCs nor players.
The variety in appereance and techs is to prevent all rangers/elars/scouts to have the exact same bow.
The low bonus compared to a crafter bow would leave some players thinking "why bother, I do fine with the bow I have" - this requires also the quest to be long, hard and very, very time consuming. This is not an item for the casual player, sorry, this is an item for the best of the best.

Crafters will say "but that is bad for us, who will buy our bows then?" - My answer - when has a lvl 100 ranger with his PS 2*socketed fine yew composite longbow ever asked for a new one (unless he's wasted his 2 sockets of course).

Another option could be to make the quest only available to pure rangers (or duo-classes)- as to give the singleclassers a bonus that is comparable to the bonuses the massive multie'd has.

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Make weapons dyable or customizable in other ways. Look at Andúril, Aragorns blade in Return of the King for example.
Firstly, it would add to the feeling that you don't have the same wep as all others, even if it IS the same. And it would add a most needed "extra" for fletchers and weaponsmiths, who right now are "behind" armorers/tailors concerning crafting options.

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If not the quests for the class weapon, then add some _extremely_ rare formulas for, well, let's call it epic items. A crafter level of 100 (soon 120) required, and a adv level of 100 (soon 120) to use.
"Well, they won't be rare for long" - Yes, because the form would have limited uses. The point would be that if ranger gets form for "Legendary Bamorian composite longbow", he'd give (yes, give) it to a fletcher who makes it and gives the item to ranger. After that, the fletcher would be able to make one or two more bows of the same kind before the form would be auto-deleted (still, the reward for ranger is a new bow, the reward for fletcher is the ability to make one to a friend, or for sale). When you then consider the rarity of the form drops (they should be so rare that once you find one you will really feel as if finding a unique item).

To prevent camping, allow the forms to drop from any kind of mob of a specific rating (120-140 perhaps?)

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Constructive critisism is welcome of course - this is just what I would considernice treats for the highlevels, and something that would make me really look forward to being lvl 100 instead of justthinking "I wanna be 100 so I can level my second class to 100 and then, uhm.. kill stuff..) :)