You can treat Resonance as though it read "Epic" (but better--you get copies during everyone's turn), and more than any of the other cards, this gets a lot of value playing it that way. Just make sure you have a win condition that operates with nothing but lands, and you're set.

No need for a win condition. Play this, your opponent will punch you in the face, scoop, punch you again, and storm off.

Seriously. This card is too annoying to live.

I think that's a wording error on my part. Resonance is only meant to occur once.

I put resonance on this card not for this set but for casual purposes (persist and undying immediately come to mind).

You might want to add "If you cast this spell from your hand" like on Rebound.

Assuming an end step occurs do you think this really problematic? The only instances I can think of that would be problematic are flickering a Snapcaster Mage and using the ability of my iteration of Ashiok I created for this set Ashiok, Phobia Monarch. There's probably a few things that would allow you to exile this from somewhere and cast a copy of it, but I think Resonance is pretty restricting in that you can't cast anything else in order to resonate.

I really like Resonance as a mechanic. I'm unsure about the reminder text wording though. It feels off.

It just seems like a fiddly version of rebound, to me.

Resonance was created for oomph, when you really couldn't do anything else in the turn the card was cast, hence its restriction.

If it only goes off the once (extra): Why on earth is "Destroy everything! Then, destroy everything!" an actual useful card?

I mean, it's kinda funny and cool seeming; but is it actually ever gonna be useful?

I guessss you can cast it before combat, to make the opponent just end-turn rather than groaning and casting a creature?

As stated before, this was for cards not in this set, hosing things like persist and undying

"Things like persist and undying". Which I think means pretty much "persist and undying". There must be only about one or two other cards total ever that would be hosed by this.

Hosing a couple of old mechanics doesn't seem worth paying 8 mana to wrath. Compare with Shadowmoor block's actual wrath, Hallowed Burial, which hoses those mechanics all at once and costs only 5 mana. Or Kamigawa block's way of hosing soulshift, Final Judgment. Either of those would be more sensible reprints than an 8-mana wrath with a "delayed boom".

I had to up the cmc due to it being an instant. Rout costs 7 for instant speed and Fated Retribution is an instant at 7. As this hits like Planar Cleansing I figured 8 was the appropriate CMC.

8 mana seems right for instant speed Planar Cleansing, but 8 mana board wipes are probably too expensive to see play.

Hmm. Yes, okay, I can see the argument for how this ended up at 8 mana. It is still unfortunately 8 mana, which is painfully expensive, but I can see how and why you got there.