I think the game For Honor is the best example of a perfect combat system that best matches this games style of Gameplay.
The Differences here is that SP's doesn't have the option for:
*Hyper-Armor (for the more Heavier and slower Characters)
*The option to Block as you mentioned which in For Honor attacks can be angled around someones block and Vice-Versa.
*Dodges CAN help you escape almost anything if done with precision.
The only thing you said that sounded kind of Iffy was the Over-Stressed part. Already in-game when getting shot while having Aleph, will causes anyone to Over-Flow uncontrollably and blow up.
But anyway Mercury wants to keep things "Unique" which is all well and good but I think might be unnecessary. Batman Arkham Asylum introduced a Unique combat method that was Successful, and since than you see it in other games like the Mega-Successful Spiderman. Naughty Dog’s Uncharted series borrowed Heavily from other games and blew up cause of it. Darksiders borrowed from God of War and The Legend of Zelda and they have a trilogy so far.
My point here is I don't think gambling it all on making things Unique is always the best way to go but if there going to stick to there guns then lets keep brainstorming.
Rock-Paper-Scissors is the idea right? So something like Engelsgaard said Rock-Paper-Scissor-Nuke-Cocaroach.
If blocks beat melee than Grapples should instead be a guaranteed kill for Blocks. Strikes should stagger out Grapples.
Block > Melee > Grapple > Block
But get this…Dodges are universal. (Nani!?)
You can dodge out of a Strike as usual as well as smoothly dodge around a Grapple like in Bloodborne.
How do you beat Dodges? Why you shoot the hell out of them is how. Dodges will have an increased range to them to prevent striking or grappling from them as your just shy of hitting them back, cancelling out dodge melee chains.