You're sort of missing the point. What separates an agro combo from a dps combo at a fundamental level? Nothing at all. When halone and royal authority were separate skills all that changed was one traded a few points of potency for an agro mod. You wouldn't be doing any more managing agro because the math nuts would just figure out exactly how much a agro a dps can generate over time and create a tank rotation based on that. Even if you added complexity to agro such that it clearly separated the good tanks from the bad all you've actually done is changed the number the tank is trying to maximize. In a way you could just change the label on the tank numbers of on parses from "dps" to "agro" generated. You're still ultimately performing the same actions a dps does only changing what you call it.
What I'm trying to get at here is that you're barking up the wrong tree or perhaps you don't actually want what you think you want. Perhaps you just want something more complicated to do during fights as a tank. That's fine and I agree they could do some more with tanks. On the other hand if you really want to separate yourself from a dps we're going to have to dive down a rabbit hole.
SE is catering to the player who just wants to be an all powerful god. That's why dungeons are so easy. Most video games do this, they don't ask for much real skill but make you think you're doing amazing. When you play something competitively like a fighting game you realize just how much games have been lying to you. Unless SE were to essentially say casual players can't tank, you'd get a lot of skill bloat. There are ways to make a tank feel like their role is reducing damage and controlling the mob more than dpsing but it would require a radical redesign of the tanking mechanics of this game. Most games go with the dps with armor route because they play it safe and this game plays it safest of all.
Going for the realistic here I'd like them to do with passage of arms type skills. It would be mostly window dressing but it would feel a lot more like you're actually defending your party.