A couple new items and some tweaks directed at tanks
Trial of Targon
Cost: 1500g (500g)
Recipe: 
- 400 Health Unique Passive: Reduces incoming physical and magic damage from enemies by 5%. Damage reduced by this item is tracked, and after blocking a total of either 750 physical or magic damage, it will become possible to purchase an upgrade to this item in the shop. Limit to 1 Trial of Targon item.
Visually, this item would be a large belt adorned with Targonian symbols and with attached weights hanging from it. Lorewise, this is a weighted belt that Targonians wear while training endurance or combat in the mountains, such as when training to ascend Mount Targon.
This is intended to be an item for tanks who wish to invest in lategame power fantasy in exchange for sacrificing power earlier. It requires a 1500g investment into an item which cannot be upgraded until the player has taken a total 15,000 pre-mitigation damage from a specific damage type. This isn't a particularly strong item in of itself, only granting 20 more health above its component and a passive. For comparison, 
gives 5 armor, 20 ms, and 12% incoming basic attack damage reduction for the same upgrade cost.
The upgrades to this would be "capstone" items to complete a lategame tank build, equivalent for a tank as 
is for a mage.
Path of the Protector
Cost: 3540g (600g)
Recipe: 
+ Trial of Targon (with 750 mitigated Magic damage) +
+425 Health
+80 Magic Resist
+20% Bonus Magic Resist
Unique Active - Stormguard: Create an aura around you that lasts for 5 seconds. 25% of incoming AOE magic damage dealt to allies within the aura is redirected to you as a bleed effect over 3 seconds instead.
Limit to 1 Trial of Targon item.
Icon looks like a circlet and billowing robe being struck by lightning. This is intended to be one of the most slot efficient items in the game, justified by its high cost and additional condition to obtain it. A tank opting to build this would shift a lot of their earlygame power into having a lategame power fantasy and it would only be ideal against a magic damage heavy team composition. The counterplay against this item with MR stacking should be to have a physical damage carry demolish the tank. With at least one ADC per team being almost universal in the meta, that shouldn't be too much of an issue.
Adamantalus
Cost: 3700g (600g)
Recipe: 
+ Trial of Targon (with 750 mitigated Physical damage) +
+425 Health
+80 Armor
+20% Bonus Armor
Unique Active - Fortress: Gain a shield that blocks physical damage equal to your bonus armor, lasting up to 5 seconds, and apply a 20% movement speed slow to yourself. When the shield breaks or ends, the self slow is removed.
Limit to 1 Trial of Targon item.
Icon looks like a masterfully crafted and heavy full-face helmet and platebody without any blemish. This will clearly be the more commonly built upgrade considering the prevalence of physical damage compared to magic damage due to differences like average champion DPS levels, turret damage type, and monster damage types. At 3700g and with additional conditions, this would have the highest opportunity cost to build in the game. The counterplay against this item with armor stacking should be to have a magic damage carry demolish the tank. It's intentional that this would strongly punish team compositions that are heavily AD skewed. Later in the thread I have some suggestions to encourage more DPS APCs in order to make it easier to draft a composition that won't leave your team skewed to only one damage type.
Frozen Heart has the issue of invisible power causing it to feel weak. I suggest reducing mana and passive aura size in exchange for a new active effect. The active effect would summon a temporary blizzard with a stronger aura effect. The blizzard aura would ramp-up and grow in radius over time before fizzling out. The active effect can be stronger than a passive aura because its strength is windowed.
Cold-Steel passive on Thornmail has the annoyance of anti-synergy with building 
together with it, and in some ways has anti-synergy with Thornmail itself due to less reflected damage from slower attacks. Building both Thornmail and Randuin's feels obligatory for a tank against any team with more than 1 crit user, such as a bot lane marksman along with any of the following 
, yet a tank will already be shredded by such a team so the anti-synergy on these items seems uncalled for. I suggest removing Cold-Steel on Thornmail and replacing the 
in the recipe with with stats adjusted accordingly.
Gargoyle Stoneplate is rarely used and overly niche. I think that it could be given in its recipe (think of it as trading components with the suggested Thornmail above) and be given the Cold-Steel passive. Cold-Steel is a better fit on this item and can allow Gargoyle Stoneplate to be a good item to build against hybrid on-hit champions in addition to its current niche for teamfight engaging. I think the current Gargoyle Stoneplate effects should also be reworked to something with a similar intended result but better clarity for players and less strange abuse cases/interactions (old interaction with Cho'Gath for example). I don't have any specific suggestions for reworking the passives, but maybe someone else would like to give a go at it in the thread. I think there's a cleaner way to accomplish what this item was intended to do.
I think that Warmog's should grant greater slot efficiency in exchange for a much higher cost. I think that Warmog's + a completed Trial of Targon should be equivalent to + 
for a mage or 
+ 1.5 completed Zeal items for a Marksman.
Overlord's Bloodmail for Summoner's Rift with an entirely different effect/purpose. Built out of 
+ 
, it would be a Magic Resist equivalent to 
that steals a minor amount of health as a healing effect from units damaged by Immolate. This would provide top laning tanks a rush item to deal with AP poke champions in top lane, like 
and other mages 
who go to top lane to bully melee champions. With this option in place, it might be reasonable to buff these champions to shift them into other lanes.
If armor is buffed for lategame as in some of the earlier suggestions, then magic DPS should be a good counter to deal with armor stacking tanks so I actually do want these kinds of champions to be strong, but I think poking APCs and hybrid on-hit champions aren't healthy in top lane so they should be encouraged to thrive in mid/bot instead. APCs could get some assistance in last-hitting early on without granting them the ability to poke more powerfully. That will favor competing in bot lane without being oppressive. Consider this:
Cull
(450g)
Unique Passive: Basic attacks deal +10 magic damage against minions and monsters and restore 3 life on hit. Killing 100 lane minions transforms Cull into Nashor's Nail.
Nashor's Nail
Cost: 800 (50g) [or finish stacking a Cull]
+ 
+12% Attack Speed
Unique Passive: Basic attacks deal +15 magic damage on hit.
Nashor's Nail would build into 
as a replacement for 
. This is a better fit because those items deal magic damage on-hit, not physical.
Nashor's Nail would build into 
in exchange for a cost reduction of 800g on combining, keeping the total cost the same (not accounting for Cull passive).
This would provide an incentive and crutch for on-hit APCs to be played in bot lane and focus on farming instead of lane bullying. Some typically AD marksmen could also receive buffs to their AP on-hit builds to provide more mixed damage carry compositions: 
.
Also 's turrets could deal on-hit effects upon laser charge attacks. This will open up a new build style for him as a DPS APC.