You didn’t solve the original problem that you had with it.
Remember how you made a point about how the are basically impossible to swap out? And how you had a problem with it, because it meant that even if you had something with better stats you couldn’t use it because you were effectively locked?
The solution would have been this: Make tier sets apply on gear you choose, like an enchant, you can extract the set and re apply it to another piece of gear, and instead the higher the difficulty the stronger the set bonus.
Let’s make an example, lfr set gives your main gauche a 25 % chance to proc a pistol shot, normal has a 30%, heroic a 35% and mythic a 40% chance.
If you have a mish mash of various difficulties it either applies the lower version, or it calculates the values based on which sets you have of which difficulty.
If the effect doesn’t increase damage in that way, but rather makes your ability do something else, like for example, outlaw rogue 4 set, where it basically makes you fire between the eyes after you used pistol shot 6 times, you simply increase the damage of the proc by a %, or increase the effectiveness in another way.
Or there is another option, but i didn’t really wanna mention it because it doesn’t really solve the problem.
Make the tier set work like an enchant, but you cannot extract it, but this would effectively mean that all that changes is simply that you can choose the piece of gear you wanna put it on. Which is still a pretty big deal, but it doesn’t fundamentally solve the problem, also you could get the tier from lfr and then apply it to a mythic piece of gear.
I think the first option is better.