Unstable Grounds

[Guest BGMCANN0N]

Unstable Grounds
[Field Spell]

When 4 or more monsters are on the field: Destroy this card. If this card is destroyed by card effect: Destroy all face-up monsters on the field.

[Flash Flyer - Sakura]

I guess it punishes the opponent for massive swarming, but other than that, doesn't really do much. Suppose Flip decks could use it to just reset themselves (and everyone else) when needed, but that's the only use I can see for it right now. Maybe some other options, but yeah.

 

Concept is good, but somewhat limited in application (especially since Links are a thing now and you can't flip them or Tokens face-down). At least it gets Terraforming support and all, so there's that. As much as I would say to give it a Quaking Mirror Force thing, that'll kill off your options more if you had stuff on the field (but screw over the opponent if you didn't have much anyway). IDK, maybe add that condition until the End Phase?

[Eshai]

Metaverse turns this into something similar to Needle Ceiling, so yeah maybe Chain Beat would use this. I could see this working, although I would make it more like Quaking Mirror Force where the monsters can't be flipped back face-up. I will say though if this was made for Link Format then this card would be kinda sorta not worth it, especially when you specifically have to use Metaverse to make it good, so might just want to make it a Continuous Trap.

 

Or you could rather make so that monsters that don't change their Battle Position are destroyed/banished instead, which would make it A LOT better. 

 

You might also consider adding a OPT or GY effect similar to Book of Eclipse but backward, where you can flip face-down monster face-up to draw a card.