"He's a silent guardian, a watchful protector, a Dark Knight." ~Gordon.
First, I have no idea where to post this. It's not a real suggestion, but an idea that needs some discussion, even with other tanks, also I don't want it to disappear in the mega-thread about balancing, so it's up to the mods to put this on the right place I guess. 
Intro (read first)
I got some ideas for one of the coming Warlock classes. Since tanks didn't get Warden/Knight, Knight/Warden and Knight/Warlock as new tank classes through the process, I would suggest to change the upcoming Warlock/Knight to a medium range magical plate tank. Many of its skills can be used to actually tank if it had enough aggro to sustain and a good tanking base values, which come by usage of plate armor. Additionally it would use a staff and some skills would require a staff as well. So plate users only need to build one additional weapon, which is totally fine I guess (bcz there are many new classes that require a new weapon).
Magical range tank sounds pretty overpowered, but I also got some ideas to make the gameplay much more challenging by adding an important ressource sustain management and positioning.
The skills cost focus and percentage mana to enable some sort of focus management and make it use the focus regeneration skills. Additionally you can use the Lv 35 Elite to regenerate your mana for some focus. This needs some balancing I guess, to find the break even point for an active usage of sustain management, which makes this class really powerful.
You need Willpower Construct for the full potential of this class, which needs some timing to run. Also you can't use a wand and talisman for an insanely high amount of aggro, so you need to use your skills instead.
Also Warlock brings some useful ISS, that protect the party or buff the player himself. One skill consumes HP to speed up the character, which can be used for pulling, but needs to be used carefully of course. So the class benefits a lot from ISS, but needs additional management of time and ressources. You need to decide what to do: Deal more aggro or support better. Both at the same time won't work very well.
Elite Skills
15: Silent Guardian (passive) - "Your Shield of Discipline no longer requires a shield, but a staff to be used. Additionally decreases incoming magical damage by 20% while buff is active."
20: Dark Hero (passive) - "Changes your holy strike into a magical range skill. Also reduces your Beast's Roar cooldown time to 15s."
20: Holy Pulse (replaces Holy Strike) - "Deals 1000 dark damage and adds a holy seal on the target (as holy strike does), each skill level increases to aggro." - 180 range, spamable, 2% mana + 10 focus cost
25: Watchful Protector - "Changes your Warp Charge to addinionally increase your generated aggro by 100% for 20 seconds. After usage this cannot be triggered again for 15 seconds. Each skill level increases to aggro." - 200 range, 10s cooldown, 5% mana + 20 focus cost
30: Unholy Staff (passive) - "Increases your aggro by 30% of any dark damage magical skill, reduces damage taken by 10% and increases your received healing by 20% while having a staff equipped."
35: Dark Echo - "Calls the darkness to help you gain your strength. Consumes 50 of your focus and transforms it into 50% mana." - 25s cooldown
40: Heavy Sorcery (passive) - "Allows you to equip plate armor and increases plate armor defense by 50%."
45: Bat's Shuriken - "Throws shuriken of dark magic to deal 4 times 1200 + 0.2 x STA dark damage to all targets in front of you. Each successful hit places a debuff 'Shuriken Strike' on the targets to decrease movement speed and attack speed by 30% for 10 seconds. If a target dies while debuff on it, it decreases the cooldown time of this skill by 2 seconds." - 120 range, 30s cooldown, 10% mana + 10 focus cost, 4s channeltime
50: Knight's Armor - "Consumes 3 PSI and wraps you into a magical armor for 60 seconds that increases your defense by 25%, generated aggro by 50% and reduce focus cost by 20%. Every 5 seconds you will release a sonar pulse that hits friendly allies in a 100 range that reduces damage taken by 15% for 5 seconds." - 60s cooldown, 3 Psi cost, only usable in willpower construct
60: Dark Pulse - "Releases a pulse of darkness to all nearby enemies to deal 2200 + 0.4 x STA dark damage. Each skill level adds to aggro." - 120 range, 6s cooldown, 20 focus cost, requires staff (might be equal to Whirlwind Shield of Knight)
70: Dark Punishment - "Punishes the souls of the opponent to increase it's damage taken by 2% for each consumed Holy Seal. Lasts for 15 seconds." - 120 range, 30s cooldown, 20% mana + 10 focus cost
Other useful skills for some syngery
The Mind Barrier can be used at some point, that protects allies around while in willpower construct. Beast's Roar is a cool aoe slow, that works perfectly with the "Bat's Shuriken" Lv 45 Elite, that is a channel skill. So Mobs will need some time to get close enough to interrupt you. Also Beast's Roar is changed by Lv 20 Elite Dark Hero.
In combination with Lv 40 Elite Heavy Sorcery, Enhanced Armor increases your defense you need.
You are also able to battle rez, but this consumes all your PSI, so it needs to be used carefully since you need your PSI most of the time.
Explanation for some Elites
40: This makes the class a plate tank, since I don't like mixing roles and gears. So the class would be a plate tank with a staff, that gets some boni to compensate the missing shield.
45: This should work like the Shock Strike of Champion, but as a channel skill that can be interrupted. So the tank needs to know how to move and where to stand while using this.
My fears
The class maybe needs a taunt for single target and AoE as well. Also this class could lack on aggro, because the player literally has no magical attack power. There are a looooot of ISS and utility to increase dark damage, but I have no idea (not at all) if that's enough. This needs some balancing for sure, but I think, any class needs it, so whatever. 
Other ideas
Maybe some more dots can be included effectively to enable additional dot gameplay, for example Soul Pain that reduces target's defense by 5% or something, because Warlock is also a supporter. But I wouldn't give this class too much support, because other classes can and should do better.
My expectations
To summarize, this class requires some skill to play. I don't think, a kitty rota would be recommended for this, since most skills should be used in different situations. Playing this requires a good timing, positioning and movement. Warlock benefits a lot from his own buffs and PSI management. Also focus and mana sustain is very important for this class. So you need to take care of 3 ressources, buffs, cooldowns, positioning, crowd control and much more. That's why this class sounds like the perfect tank - and it truly is, but as mentioned, requires a lot to succeed.
Discussion required
Now I would like to see some other points of view of different tanks (and also other players ofc)! Would a class like this make sense? Would it be overkill? Let me know! 
Please consider, all the cooldowns, values and stuff isn't final, it just should be a concept and show you guys, how I imagine this class. So I don't care if its 1000 dark damage or 900 and such...