Making Legendaries guild-bound is no better than having them as player-specific. You're just shifting the loot drama from the early stages to the completion.

Honestly, there's only two solutions that have a promise of succeeding:
1) Make them individual, so that each person is working on their own without conflict. Flaw: everyone will have one.
2) Get rid of them. Flaw: nobody will have one.

Honestly, each time a Legendary comes out, I either feel sad that I can't have one, anxious that I might not get it, or (rarely) happy that I'm the lucky one. Hint: that last emotion is rare.

On the other hand, Legendaries would work much better if they weren't loot/gear, but trophies of guild successes, particularly paired with Guild Halls. How awesome would it be to have a (real) Fang of the Father hanging on the wall, with a readable plaque displaying the names of the people who brought him down? Every important lore figure could have a trophy for defeating them, and as the guild mounts up their successes, the Hall shows more and more trophies.

It seems far better than a weapon that everyone contributes to, one person uses, and eventually gathers dust in some dude's bank.