Yoship seems to really care about and abide by the 'class/role equality with play style to differentiate' thing so I don't think any mage tank advantage or disadvantage in capability is gonna fly, even if we have the pld vs war or any other dps vs dps issue atm. He already streamlined damage types as evidence. Any existing damage 'types' can only boost itself but never have a base advantage from what I can tell. Everything starts from 0 and a player can either buff himself or debuff an enemy but never start at +1 or above, like monks having a starting advantage over other class when fighting bone undeads or dragoons having advantage over flying enemies (correct me if this kind of thing exists in ARR). I think this also already applies in reverse to some degree; we can barely modify our elemental resistances (I've never seen someone say that it's worth it to use those potions) and there's only apocatastasis and traited protect; no monsters have an advantage over players with different jobs or armor types. They're almost all just labels at this point.
So I dont think yoship will ever say "Lets make a tank that is weaker at tanking physical damage compared to other tanks and have him be really good at taking magical damage so some dungeons/fights people would want pld/war, some dungeons/fights people would want this magic tank". Especially when some attacks can be vague and needs testing and checking to find out if it's magical or physical (too much stress, you know?). I can already see him not liking that already, judging from his past answers for questions about elemental wheel being removed. I feel that, at best, if we're going to get any caster tank, is yoship aiming it to have the same tanking ability as the pld and war and have it revolve around magic barriers that works for any type of damage for mitigation. Something like stoneskin and sch shields, except better and for the tank's personal use and maybe a different way of generating enmity (ooh maybe party wide barrier cast that transfers/steals enmity back to the caster tank if a monster hits a party member).