The has a lot of 'Whisper' cards, which start out a little weaker than usual but can be given a permanent Boost by placing a card from the top of your deck under them
Sacrificing a land can be bad, but Id expect a green deck to have plenty of spare lands by the time youre recurring this elemental dragon
When Thrasios costs to activate, the rate at which it puts lands into play is just too slow to meaningfully accelerate the average number of possible activations per turn cycle
Gods Hall Guardian is expensive, even casting it with its Foretell cost, so you probably wont see much of it
Having the ability to flicker everything you control is incredible and becomes unbeatable in the later stages of the game when you have plenty of permanents in play
Heres a non-comprehensive list of cards that catch my eye with multiple black symbols in their mana costs: Last time around, Mono-Black Devotion didnt actually have a BB-costed two-drop, so its certainly not a requirement to have one, but it would be a nice complement to the deck (especially if we consider were very likely to get at least another playable card with devotion to black)