I believe the "18th realm" is the Merged Lands itself

If the writers come out and say, "Oh, there’s actually one more realm we added just for laughsies" we’re definitely coming after them with pitchforks. I don't think that's what's happening her, I believe this 18th wooden plate actually represents the Merged Realms itself. The reason it’s flickering red is that, the Coalescence itself is a cosmic mistake, It's the very last breath the universe takes. Is it a perfect theory? Eh, I don't donow, it has some flaws. My guess for the other 17 plates...