There are lots of threads looking at One Tamriel overall but I want to concentrate on one issue I see and a solution for it.
Crafters don't just craft for themselves so they need access to mats and runes of all levels. Currently that's simple: if you need to farm iron, jute and rawhide to craft low level gear just go to the beginning zones and do it there. But in scaled zones like the Gold Coast we get 50% mats that match our character's level and 50% that match their crafting level. If the whole world is scaled, will it be impossible for a CP160 master crafter to gather lower level mats?
The solution for this is simple IF the devs think about it ahead of time. Just leave the nodes as they are now in our alliance's non-vet zones. That way an AD character can continue to get iron, jute, and rawhide in Auridon, a DC character can get those in Glenumbra, and an EP character can get them in Stonefalls. When gathering in another alliance's home zones it would scale just as it does now in silver and gold zones. Plus there are a growing number of scaled DLC areas like Wrothgar, Gold Coast, and Hew's Bane where the growing population of CP160 characters can always farm for mats.
One other issue is that if we no longer have 3 instances of each zone we will only have 1/3 as many nodes. Because the silver and gold zones are so empty many of those node go largely unused so that's not as bad as it sounds, but it will make for much more competition for the nodes that do exist.
Again the solution is simple if implemented: double the number of nodes or at least double the yield at each node. I prefer more nodes because that would make it harder for a farmer to monopolize them all. OTOH more yield per node would reduce farming time.
There are a lot of other, more difficult issues with the One Tamriel concept and they all need to be addressed before implementation, but with a little consideration and planning this one can be solved.
ContraTempo
Carpe DM
Seize the Dungeon Master