IIRC, Druid tanks essentially got that in bear form back in Wrath, so they could use necks and rings and cloaks with dodge on them. But I don't believe they do that anymore; a quick look over loot lists suggests that bears (and brewmasters) stick to agility gear entirely, and find other ways to use the stats found on that gear to aid their tanking. It wouldn't be difficult at all for Blizzard to do as you suggest, especially if they want go for a symmetry of 3 shield-bearing tanks and 3 without.Shamans, at this time, get 2AP from 1 agility, and they can aswell convert strenght to AP, but only in a 1:1 ratio. My sugestion was just allow them to convert strenght in a 1:2 ratio... Its the easeast way to allow them to tank, because they could equip all current tanking gear... Being the exception plate gear.
But I hope they would do something different for a potential shaman tank class. As I understand it, Bears and BrewMs like crit on their gear. Perhaps these potential shaman tanks could focus on haste, much like paladin tanks currently do. Assuming stat priorities don't change for the others, that could more or less create two crit tanks, two mastery tanks, and two haste tanks, which is also a sort of symmetry.
(However, I pretty much expect that tankadins will be overhauled again in the next expansion. This whole haste stacking thing seems to have surprised Blizzard, so if they've decided it's wrong for the game, that's when they'll take it away.)