Well if you want to go into what's needed, you have to look at it from every angle.
In terms of classes, Tanks = 2, Healer = 1, DPS = 5 (Need a new healer Class)
In terms of Jobs: Tanks = 2, Healer = 2, DPS = 5 (Fairly well rounded, but DPS is odd number, making it an even seems like a good idea)
In terms of battle ability: Defense (Tank) = 2/2, Healing = 1/2, Physical Melee = 2/2, Magical Range = 2/2, Physical Range = 1/1 (Need a new physical range character)
In terms of demand: Tank
In terms of what players Play: DPS
In terms of Discipline: Disciples of War = 5/5 Disciples of Magic = 3/4
(the ones with #/# are Class and Job respectively)
Making a new tank has many what if scenarios and possibilities. Players will try it just to try it. This doesn't mean they'll dungeon with it, they'll just try it out. Could run Fates with them for some quick EXP. Though it may or may not fix the Tank issue. It will die out fast, especially if the Tank isn't vastly different from what we already have, which isn't very likely to happen. Will more people play tanks afterwards? Its not a question anyone can answer honestly because no one knows. Based on role, some people just won't play it for parties. Based on class, some may play it for the class, but at the same time just as many or more may not play it because of the class or even lack of diversity between it and the previous tanks.
Really, the chances of adding a new tank fixing the human error on the player's part isn't very likely and it does seem to have the same chances of the new tank class being used only for a short burst and then quickly forgotten or existing tanks changing to that new tank for playability, which does nothing for increasing the numbers. While I say its small, this is mostly personal opinion based on what I see daily both on forums and in game. Tank is just the least fulfilling class and you'd have to pretty much like the Tank before hand or be of a tank mindset to really want to stay as the class as a Main profession.
New Ranged DPS isn't something to completely ignore even with the Tank issue. Has potential to take charge over Bard, but Bard has some nice support and really is a support based class more than a damage dealing class, not that they cannot deal damage as I have heard the stories of solo Barding Primals.
New DoM is a personal favorite of mine and something I eagerly await, even if its not until Patch 2.9.