I never understood FON, aside from using it with singed. It gave the highest MR in game to shrug off burst AND paired with health regen? If you're investing in stats to confront loads of quick, burst damage why waste resources in long term sustain as well? Those are 2 paths in completely different directions.
And people always throw out the concept of "but it has move speed!". I don't play bruisers whose kits cannot already tag and single out carries.
FON just guaranteed that I could ignore whoever built it.

fon with singed. it was part of my awesome build

but say you're garen. you need mr because the enemy is AP heavy.
ANY item with AP is absolutely useless.
runic bulwark is SO expensive. granted, you get what you pay for, but still. sometimes you don't have the gold to shell out for it

and if you're DON'T have a heal, spirit visage is not really meant for you. singed does not have a heal.
it's great for cho, vlad, and mundo. but someone who doesn't have a heal

my big deal is the MR. the health regen was nice, but warmogs has that too, and the movement speed was nice too, but the new upgrade for the boots works plenty fine.

it's the bloody magic resist for those insane bursts late game.
i want an item that's purely defense. one that doesn't water in with armor like runic bulwark or ga, because we already have those. One that doesn't throw in AP or AD because those can't be used for certain champs.
I want one that gives over 50 MR. whether it has health or health regen or something, i don't care, but a purely tank item is something i desire.

I don't think you understand the value of Twin Shadows or Shiv or even Zephyr. Twin Shadows is a VERY fine support item. If one gets it as the 3rd or 4th item after sightstone, philo stone, Kage's, and aegis (granted this is all what I like to do), then going to Shadows can be really good. By the time you get to the point I like for TS laning is either almost over or done and the wraiths are a huge boon to scouting and soft CC for the beginning of team fight phase.

Stattik Shiv is for ADCs that have something of an average mid-game like Ashe or Trist. They are champs who like the AS item after either IE or BT, but don't really need to PD to be as effective. It really helps early mid-game lane clear and the fact that it can crit is decent on Ashe because of her passive, even if it's not the strongest passive. Again it mostly exists to help out champs with weaker mid-games.

Zephyr is the toughest one to play because it does a lot of things that seem to already exist. About the only champs that I would run Zephyr would be Irelia or maybe Garen because it has just enough different utility to help after one gets the core item of Trin-Force or BC and GA. It's a very late-buy item to say the least.

The reason you want the ability to shrug off burst while still having sustain, is not for bruisers.... I really wish people would learn the difference between dedicated TANKS and BRUISERS. Bruisers are designed to get more kills while surviving. Tanks are not primarily designed for kills. We are designed to survive so others can. As such, we need not only the ability to survive that initial burst, but with the proportionately less gold we get, we also need to be able to survive a teamfight and still go farm a bit before we can afford to go b and shop. The others all get kills, while the tank receives only half the gold even if we were crucial in getting those kills. There has been many times where my team wouldn't even follow me into a fight until i died and took the hp on half their team down first. Without my initial damage output no one would have gotten kills, yet i receive half the gold and am expected to buy itmes to counter things that generally cost less then the counters themselves.

And as for movepeed. Your argument is once again about bruisers. Most of the complaints aren't about bruisers needing movespeed. It is about tanks. Bruisers don't need movespeed because they are designed specifically for killing squishies. Tanks on the other hand, have always received the short end of the stick when it comes to the attention they deserve. Even in most normal and low elo games, you see more Malphites and Cho'Gaths and Garens than you see Leonas, Maokais, or Tarics.

Xin loves Zephyr. I like to jungle as Xin and this is the first thing I buy after Wriggle's. It has movespeed for ganking and positioning, it has a little damage, and has AS which is Xin's best friend. And the CDR synchs up well with his q reducing CDs by 3 seconds. With just Zephyr and the 3 second reduction from q, everything but his ult is roughly 3.5 second CD.

How do mr and health regen grant bruisers kills? In that brief window between an apc's first and second kit rotation, you better have enough dps to kill them. FON is just too expensive, considering you also need an additional health item on top of it to really get the most out of it.
And if you know how to tank, you know the scaling value of actual health with resistances. Health regen just does not scale into the end game period.
I will say FON was viable only if your opponents were stacking health% on hit...

They removed MS form tank items because MS is the strongest damage mitigation in game. If they can't catch you they can't kill you.

FoN was good in some cases, I agree...why remove it?

I never said MR and HP regen help bruisers. I said they help tanks. And I am well aware of scaling with health and resistance, that is what Warmogs, Raduins, and Sunfire cape are for. Your argument is that you need health to combine with FoN, well, no duh... You still have 4 item slots ON TOP of FoN. FoN, Sunfire cape, Warmog's, Raduins, and Merc Treads. Gives you all the HP you need for the scaling and % hp regen on FoN. AND you have another item slot open for specific counters. Even with the old items, Warmogs was 920 without stacks, Raduins= 350, Sunfire Cape= 450. So, 920+350+450=1720. With, say Mao as an example, a base HP of 2041. 1720+2041=3761. FoN gave .35% of max hp per second. 3761*.0035~13 per second or 65hp5. Doesnt seem like much, but combined with the base of Warmogs at 30hp5 and Mao's 23. That is 118. or almost 60 hp per second. Tanks are IDEAL for those after team fight pushes with FoN. Team fight ends, most have to go b to heal. But with 60hp per second you no longer need to worry about that. You argument is based ENTIRELY around the fighting itself. You are not considering the out of combat advantages.

The remove of Force of Nature has left a gaping hole that is not currently filled by any item.

What do you buy to give Garen, Mundo, Shen and other such non mana reliant tanks a big Magic Resistance boost now? Banshee's veil gives no HP regeneration and wastes gold on mana. Abyssal is useless for melee heroes unless they are AP carries (Fizz, Katarina, Eve, etc). Hexdrinker is designed for AD carries not tanks. Quick silver sash wastes a slot as its more of a early to mid game item. Other items such as mercury treads, and the various other items that build off the 400 gold MR cloak only give a minor boost in MR. etc