ok so here's the thing having played a lock most of the way through-out WoW's life time gotta point out they have got to tank a raid level boss every xpac aside from MoP in which we finally got almost a full toolkit to do so, now a lock in DA within a week ok MoP going life tanking Heroics was possible even without the 6% crit immunity, that being in mostly greens, and a handful of quest blues possible, basically entry level to queue.
Now having scoured MMO-champion for PTR notes a couple come to mind that really had they been implemented would have made lock tanking a thing.
Archimonde's Vengeance has been redesigned each time the Warlock or their pet takes damage, the Warlock now gains 1.5% of the unmitigated damage back as spell power for 20 seconds.
Give back Fel Armor now reduces damage taken by 10%
undo Fel Armor now grants 10% health, rather than 10% Stamina.
Nether Plating give us back the 6% crit immunity
with those the only thing really missing is a second Active Mitigation option cause we all know that Fury ward would be boss with vengeance, hell with soul leech you could amass a fairly sizable shield fairly quick with Molten Core procs, but really we would need a % based skill to balance things easier ala shield block, Shield of the Righteous, savage defense.
Curse of Enfeeblement = weakening blows
Unending Resolve = Divine Protection only better 40% DR or and additional 10% all the time
Dark Soul: Knowledge + 30% mastery which ends up being 20% around reduction = Ardent Defender minus the non gibbing
Provokation = taunt at the change of a flag
Fury Ward = Shield Barrier sp/ap = shield
Demonic leap = heroic leap
Hand of Guldan = Consecrate
Corruption = Deseases,
500% threat mod
So at my count we have 2 anti insta-gibber, we have 500% threat, we have 1 of 2 Active mitigation, we have quick travel, 25% base Damage reduction, a tanking rotation
now what we need something to match the dodge/parry/Block % difference base being around 6% while we have 10% healing buff (so possibly a increased healing factor based on spirit), a second Active mitigation choice. i cant think of any thing we are missing after that, this is all before talents and glyphs so to be thorough
tier 1
2/3 can be used to tank
tier 2
3/3 control or heal tanking
Tier 3
3/3 tanking usable
Tier 4
2/3 tanking useful
Tier 5
1 pretty much best choice 1 usable
Tier 6
1 choice here with the changes show
Glyphs
4 out right tank choices
eternal resolve
siphon life
healthstone
Demon hunting
unending resolve
this concludes rant i count 2 things that need addressing that they don't have code for so i would put it around 70% done last 10% being balancing