Legendary Fishing Pole -- I love this idea *so much*. I'll expand on it:
Since it would have to be accessible for everyone, it would be preferable to have the base item either be fished up, or created as an epic item from Archaeology. Either way, it would have to be able to be actively obtained, and not a drop from a Bag of Fishing Treasures.
Components could be fishable, but just to keep it legen-wait for it-dary, the fishing areas would have to have some level of difficulty to get to:
PVP: The waterfall in Battle of Gilneas.
PVP: The moat around Blacksmith in Arathi Basin.
Raid: Serpentshrine Cavern -- Anywhere in the water.
Raid: Firelands -- Anywhere in the lava.
Dungeon: Wailing Caverns -- Anywhere in the water.
Dungeon: Maraudon -- Anywhere in the water, or even have the base item be a drop from Rotgrip.
Dungeon: Zul'Gurub -- Heck, they could even bring back an updated version of Nat's Fish Terminator, and use that as the base item.
Dungeon: End Time -- The lava near the Echo of Baine, or the pool surrounding the Echo of Tyrande.
The PVP components would need some balancing work. I can already see players trying to kick me (Reason: "Noob is afk") if I was off in the corner fishing instead of helping the team. Maybe drop rates for components from PVP areas could be a little higher for these areas? Maybe 5%? Something high enough that it could be done without having the rest of the team complain about your fishing compulsion.
The Raid and Dungeon components could be done after enough trash has been cleared to reach a fishing spot, so after completing a dungeon, you could stick around and fish at your leisure. And in all these cases, the areas should be active enough to prevent botting, or at least make it extremely difficult. Maybe throw in a mechanic that would spawn an add occasionally.
And just to make it annoying -- add in The Limnologist and The Oceanographer as requirements, too, so there would be a time component with the Winter Squid.
Fix for water walking -- and levitate. I notice that I fall into the water sometimes when mouse-moving, too. It's like the pitch of your toon gets overly sensitive, and it thinks you want to dive in.
Another Meta -- Just because we need something even crazier to get, how about a meta-achievement for winning both the STV and Northrend contests? One of the things I've noticed after winning a contest on a toon, is that I lose all interest in going back to the tournament, because I already have the Master Angler achievement. I need more motivation to go back. The meta could award a title like "the Merciless Angler", since winning once earns the admiration and envy of your peers, but winning twice earns only the ire of everyone in guildchat.