The concept simply doesn't work (and isn't required at all) in real-time which is why you basically have to resign to saying "it has to be free and instantaneous". Think about what you're saying.
"Zone of Control" style bullshit can be done in real-time, but AoO's is the most retarded way to do it (as it's just copy pasta a turn-based system lmao).
Didn't you yourself say you prefer threat-based systems? They work in real-time. I hate them because I hate anything systemic that takes away the control of my character(s) (other than spells like charm/fear/confusion etc). But they do work.
AoOs simply should not have ever been included in real-time games. There are plenty of better ways to do the same thing that aren't flawed by design.
I've already stated that RTWP is bastardized turn-based with many turn-based elements. You don't buy this, but we have been over it. That's why I have zero issues with its implementation in a supposed 'real-time' system.
Threat is a mechanic solely afforded to the player. If you give it to the enemies, it literally 'takes the wheel' from the player. AoO is entirely within the player's control to activate are not so it can be given to him and his enemies.
I like threat because in MMOs it gave melee characters and 'tanky' characters a real job to do, but it is strictly a player-focused mechanic. It's why I called it the 'ugly cousin.'
How else would you take advantage of it?
By making AoO a toggle of course. You want to use it, you turn it on. It'll then trigger automatically but it will eat your regular attack.
Your problem is that you're trying to persuade yourself that free and instant AoO is fair.
If it eats your regular attack then there is no AoO in 1vs1 situations. When the person departs, you're attacking just as you always would, no? Because you've made it "eat" a regular attack. If you're in 1v1 and someone passes through your area, you attack them, but give your opponent a free attack as well. This is a pretty broken way of implementing it. I can already foresee abuse and zaniness associated with juggling enemy regular attacks. This is not at all a good system.
Free and instant is fair because
both sides do it, and
both sides understand it's there. And like I said, there are costs, just not immediately obvious ones: melee literally has to be standing next to you for it to activate. There is a cost associated with needing to stand next to someone to get a
potential AoO, not an automatic one (after all, a character must move away or through to trigger it). You obviously didn't transport yourself to be next the mage free of charge, so why should they be able to relieve themselves of such a precarious situation free of charge? It took you time and potentially even health to stand there, did it not? While everyone else is attacking and doing damage, you are...
walking. This is why things like AoO and ZoC's
elevate the importance of positioning and buffs the value of melee. If you can cruise through melee lines untouched, melee's importance and tactical value plummets. If melee's reward for sneaking to a mage or ranger is that said enemies go into kite mode, what is the reward in that? Furthermore, craftier characters like rogues get a nice boost since their ability to navigate Zones of Control is unique. Instead of just being pure damage dealers, they can be apt at acquiring better positions. You know, tactical shit.
And I think there is far too much bitching about just how "effective" these free AoO's actually are. They're not game-ending. They're minor thumps on the head to remind you not to fuck up your party positioning. It's not like Total War where the cavalry crashing into your archers is GG. It's not a game over situation at all. It's a minor mechanic nestled amongst many others, neither overpowered or underpowered.