I have a few problems with the changes you're proposing.
1. Reducing the number of mods on equipment to 3 is something me and other players say we don't want, because it would lower the amount of customization we would have with our gear. Less slots means less build freedom.
2. By making higher rarity gear have stronger mods, you're defeating the purpose of limiting mods to three. There will still be a power difference between people with fully maxed out gear vs those with sub-optimal gear. That said, the way you're suggesting it scales numerically would shrink the power difference heavily, but mostly by buffing people in white, green or blue gear.
3. I dislike the idea of EVERY piece of gear having 3 mods, because it will make "Normal" items bloated. As an example this means that within the first 3-5 minutes of starting up the game, new players will have to wrap their head around dozens of mods because every white item they pick up has three random modifiers. Not only that, but the bonus from higher rarity items will be confusing. We still have players asking about which item rarities are better despite higher rarity items having different mod counts.
4. Now I don't know if I'm missing something, but are level 10 players going to have the same potential mod strength as level 80 players? That's going to throw balancing damage completely out of whack because you'll have level 10 players with mods that add hundreds of flat damage. You would have to do an absolutely major mod rework and damage scaling rework for the entire game to account for this.
5. The numbers you're suggesting for armor scaling are basically what they are right now. A level 10 player having about 100 armor and a level 80 player having around 800 armor sounds completely normal, unless if you're suggesting that cloth armor should have about 800 armor like metal does. The current way that player armor scales will not be very strong encouragement to replace player's lower level gear, as several full DPS sets at end game already get by with 400-500 armor, which would be level 40-50 in your example. Not only that, but if the amount of armor scaling is about the same, then your suggestion boils down to nerfing players by 3 mod slots for the most part, while Citan wants to encourage people to sacrafice damage for survivability.
6. Making every kind of armor from the same armor class give a bonus like the ones you're suggesting forces people into certain roles which I heavily dislike. For example, it's very likely you'll pick up at least 3 metal pieces of armor in goblin dungeon when leveling, but with your system that would give people +50% extra taunt which would basically force them to always take aggro from there. It's highly likely someone will be forced to accept one of your armor set bonuses even when they don't want to, since negative taunt or positive taunt can be undesirable. It's not a good idea to assume that everyone wearing 3 metal armor pieces wants to be a tank. In fact, as a tank player I would say the evasion bonus from leather sounds way more appealing than the taunt bonus.
7. For tanks, getting extra armor/health is not that important, at least outside of the early game. You suggested that tanks would want to use higher level gear as soon as possible for the armor/health for better survivability, but that is not the case even now, despite higher level gear typically giving more armor. Even right now several BiS tank equips are not level 80, and several of them aren't even metal armor, because sacrificing hundreds of points of armor is worth the resistances they give. In fact, a big reason why tanks can get away with lower level equipment is not because they don't care about getting good mods, it's because there's so few good mods for tanking that the level penalty to your equipment doesn't matter as much. For example, on Unarmed/Shield there is ONE mod on the chest that is worth getting for tanking (Physical resistance from unarmed) while everything else just gives damage, very minor bonuses, or don't really scale that much (Ex. Generic poison resist/accuracy). That one mod is really good and you want it to be level 80, so you augment it to 80, and the level of the other mods isn't very relevant since they don't contribute to your survivability nearly as much. Your suggestion wouldn't change this because even if the chest was limited to three slots, you still would only care about having that one mod while the other ones are just damage bonuses.