So been playing Alpha for almost a month now and let me tell you, ALL TANKS got some pretty huge nerfs, especially to magic resistance.
I foresee casters being the paper to the tank's rock in BfA. I have two reasons for this reasoning:
1: Like I mentioned, tank's magical defenses are getting SEVERELY nerfed. I'm talking about 1/3 of what they are in Legion. It is NOT a small nerf, it's a BIG one.
2: ALL of the tank's snaring abilities were nerfed as well. All 50% snares that are live right now are getting nerfed to 20% snares in BfA and the few 70% snares which all have limitations like long CDs (DK DnD w/talent and DH chains) are getting nerfed to 50%. ALL THE WHILE every DPS snare will be staying the same. So Mages can still 70% tanks while every tank slow will only 20% snare them.... EZ KITING.
Couple those things together and I really can't see how ranged classes will be losing to tanks now.