Tanks don't feel like 'tanks' anymore.
As a tank player ever since runes reforged it hasn't felt like I've been playing a traditional tank, when playing things like Poppy, Maokai, Nautilus, Rammus, Amumu etc. Since when was it their job to straight up kill people with aftershock or die trying. They're made to CC and tank damage not to be a killing machine(For the most part). My job was never to kill enemy squishies, it was to CC them or keep them busy long enough for my teammates to kill them. But now I can kill them without any help most of the time.
In the old Masteries as a pure tank you would either go Courage of the Colossus or Stoneborn Pact if more supporty tank. Both of these gave you actual tank stats or effects, and made you feel tanky. No offensive stats whatsoever.
However with the introduction of the new Runes I never felt like there was a rune that fit the 'tank' playstyle. Currently it feels like resolve runes are somewhere in-between bruiser and tank and not specifically for either.
Aftershock - Is in a way similar to courage, you proc it by hard CC'ing someone and you gain a small amount of tank stats for a limited time and some damage around you, on paper it sounds like a great replacement for Courage. But in reality as a tank, for me damage has always and will always be irrelevant, aftershock is more of a bruiser tank rune especially with the latest patch that added ad/ap scaling and changed the resistance values values. This is still the closest we have to a proper tank rune.
Guardian - Extremely limited in how it can be used, especially toplane. It works well for some tanks which specialize in peel but for most CC tanks you want to be ontop of the enemy squishies and not your own. This is mainly used for supports and on botlane, not really for toplane tanks.
Grasp of the Undying - You pick it to win lane and duels or simply to sustain in lane. It's a very situational rune, especially for a tank so it's not always the best. Doesn't work well with a lot of tanks either, but still an option.
We're missing a rune that fits the role of a traditional tank and can help them do their job and actually tank and CC or help with it. Something like brittle where it extends CC durations or straight up old Courage back. Changing Aftershock from doing damage around you to an aoe slow is also an option.
Some may argue that tanks should be able to kill squishies in teamfights or even solo and apply kill pressure. Read this post by Riot. "Tanks excel in shrugging off incoming damage and focus on disrupting their enemies more than being significant damage threats themselves." - Which is far from what they are today.
This doesn't mean that tanks shouldn't do any damage at all, I still think they should have mainly %HP damage to deal with other tanks and enough flat to be able to farm properly. Currently they have enough damage to kill most squishies without breaking a sweat, but in return get killed fairly quickly in the lategame, especially by adcs, which is a big problem when the games are longer. The threat of tanks should be their CC and brick wall feel, not their burst potential and kill pressure.
Generally I think tanks do and take too much damage and have lost their role as a traditional tank, they used to be a brick wall or CC machine, now they're just made for killing. I would love to see some changes to runes regarding this, aswell as maybe items.

