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Cavern of Souls should come into play tapped. It should be a Legendary Land even perhaps. It encourages Tribal decks and effectively reads, "Add one mana of any color. Use this mana only to cast creature spells," for these specific decks. Okay, not everyone is running giants, but this card makes splashing giants that much easier, especially if someone is dropping Primevial Titan first to fetch another Cavern of Souls.

I anticipate many more decks who will run:

4x Cavern of Souls
4x Heartless Summoning
4x Solem Simulacrum
4x Primevial Titan
4x Wurmcoil

And some combination of Inferno Titans, Frost Titans, and Grave Titans. Cavern of Souls is only required for the Titans, so naming Wurm or Golem is unnecessary. Never mind that Cavern of Souls is also going to be ridiculously expensive just as every other card I've listed already is. Throw in Mimic Vat and you've got something going.

As many people have mentioned, Ghost Quarters is definitely going to see more play. Perhaps I'm exaggerating about this, but people play counters to control the board and the stack. Cavern of Souls prevents control of the stack (can't counter creature spells on it) and the board (can't prevent certain creatures from entering). You can bounce things, but what does that do except delay doom? ;-P