More speculation:
It's possible professions will be completely re-worked. Currently Old Azeroth is full of old "endgame" materials which are more-or-less never used, because people leveling professions go straight to Outland, and many old endgame items are worse than Outland greens. Not to mention the old profession quests (which I suspect are only useful for pacifists...), and obscure limited-supply recipes.
Cataclysm would be a perfect opportunity to re-arrange some of that old content, perhaps to make it slightly more useful again. Critically, the way Northrend is designed is quite different from Old Azeroth - it's a lot easier to find things in Northrend, to know what you have to do without someone telling you. So if Old Azeroth is being updated to Northrend-standards, things like limited supply recipes must surely go. Likewise the Big Iron Fishing Pole is far too well hidden for a mid-level pole.
(For reference, 'fish' tags can be used to link to fishing gear, and give you a tooltip - basically any page or thing the Fish Finder search knows about can be placed in fish tags and is automatically linked. The forum simply requires the correct spelling, while the Fish Finder tries to guess if the spelling is wrong.)
Beyond that, I'm unsure. We know that the level of many zones is changing. We don't know everything, because the BlizzCon presentations only showed about half of the zones. Logically, that means the fish caught in different zones will also need to change, because there is a loose relationship between zone level of fish level. It would make no sense for a newly-low-level Azshara to contain fish that can't be used until around level 50.
But: A determined level 10 angler can already catch almost anything, so the purpose of restricting the best fish to high-end zones is less clear. In the past I've suggested that some worthwhile fish should be caught everywhere, requiring anglers that want to catch certain types of buff food (for example) to travel to specific zones. But travel isn't favored anymore (instant teleports to dungeons are the latest example). People used to flying from Dalaran to neighboring Wintergrasp to stock up on Fish Feasts, might get freaked out by traveling to Winterspring or the Western Plaguelands instead. (Members of the Linken's Boomerang Owner's Club will be less concerned.)
The big unknown is the mooted change to the fishing mechanic, which could change everything. Of course, "Blizzard" have been talking about this for 18 months (it was first mentioned just before WotLK testing), so I'll believe it when I see it.
