Have you considered Might of the Masses or Strength in Numbers?
These would go well with all the tokens and they would add a surprise factor to your deck.
Unless there is a Ghostflame Sliver on the battlefield it cannot, since Sliver Overlord is every color.
You need to target the sliver and protection disallows that.
How about adding an alternative cost?
<keyword> 2BB (You may cast this spell for its <keyword> cost. If you do, it enters the battlefield transformed.)
This opens an interesting door for future design of double faced cards as well!
And simplify the counter by using only one type:
If you cast this (from your hand), it enters the battlefield with a phylactery counter on it.
This should work since a lesser version might be preparing to become a Dracolich and has started gathering phylacteries already.
While Erebos, God of the Dead isn't on the battlefield it's a creature card (The type-changing ability that can make the God not be a creature functions only on the battlefield. It's always a creature card in other zones, regardless of your devotion to its color).
While Containment Priest is on the battlefield it would always exile Erebos if you didn't cast it, devotion has no impact at this point.
It's the other way round for Starfield of Nyx and Copy Enchantment - the card entering the battlefield isn't considered a creature until it enters the battlefield.
Soul of Krosa with provoke is a cool throwback!
However, I think planes themselves would go better than a mere continent, though Souls of <location> would give us more to work with .
Maybe something like this:
Soul of Pyrulea4GG
Creature - Avatar
Reach 3GG: Search your library for up to two basic Forest cards and put them onto the battlefield tapped. Then shuffle your library. 3GG, Exile Soul of Pyrulea from your graveyard: Search your library for up to two basic Forest cards and put them onto the battlefield tapped. Then shuffle your library.
6/6
Two issues here though - it gets wordy and Reach was already used for Soul of Zendikar.
Also, great idea about tweaking Soul of Rath! Haste would reflect how flowstone affected creatures in Flowstone Strike and Flowstone Channeler.
Soul of Rath4RR
Creature - Avatar
Haste 3RR: Up to one target creature you control and up to one target creature you don't control gets +3/-3 until end of turn. 3RR, Exile Soul of Rath from your graveyard: Up to one target creature you control and up to one target creature you don't control gets +3/-3 until end of turn.
6/6
Soul of Rath4RR
Creature - Avatar 3RR: Up to three target creatures get +3/-3 until end of turn. 3RR, Exile Soul of Rath from your graveyard: Up to three target creatures get +3/-3 until end of turn.
6/6
Some evergreen keywords might do these avatars some good as well!
Soul of Serra's Realm - Lifelink,
Soul of Equilor - Flash,
Soul of Rath - Menace?
These would go well with all the tokens and they would add a surprise factor to your deck.
Also 4 Deathrenders might be a couple too many?
And some 1 cmc cards could help out - Elvish Mystic, Wild Growth/Utopia Sprawl?
As a result, they will never return to the battlefield or trigger creation of more snakes.
You need to target the sliver and protection disallows that.
<keyword> 2BB (You may cast this spell for its <keyword> cost. If you do, it enters the battlefield transformed.)
This opens an interesting door for future design of double faced cards as well!
And simplify the counter by using only one type:
If you cast this (from your hand), it enters the battlefield with a phylactery counter on it.
This should work since a lesser version might be preparing to become a Dracolich and has started gathering phylacteries already.
It lets you play as many eldrazi as you own :D.
Oh, there's also Research // Development.
While Containment Priest is on the battlefield it would always exile Erebos if you didn't cast it, devotion has no impact at this point.
It's the other way round for Starfield of Nyx and Copy Enchantment - the card entering the battlefield isn't considered a creature until it enters the battlefield.
However, I think planes themselves would go better than a mere continent, though Souls of <location> would give us more to work with
Maybe something like this:
Soul of Pyrulea 4GG
Creature - Avatar
Reach
3GG: Search your library for up to two basic Forest cards and put them onto the battlefield tapped. Then shuffle your library.
3GG, Exile Soul of Pyrulea from your graveyard: Search your library for up to two basic Forest cards and put them onto the battlefield tapped. Then shuffle your library.
6/6
Two issues here though - it gets wordy and Reach was already used for Soul of Zendikar.
Also, great idea about tweaking Soul of Rath! Haste would reflect how flowstone affected creatures in Flowstone Strike and Flowstone Channeler.
Soul of Rath 4RR
Creature - Avatar
Haste
3RR: Up to one target creature you control and up to one target creature you don't control gets +3/-3 until end of turn.
3RR, Exile Soul of Rath from your graveyard: Up to one target creature you control and up to one target creature you don't control gets +3/-3 until end of turn.
6/6
This one also gets wordy.
Soul of Rath 4RR
Creature - Avatar
3RR: Up to three target creatures get +3/-3 until end of turn.
3RR, Exile Soul of Rath from your graveyard: Up to three target creatures get +3/-3 until end of turn.
6/6
Some evergreen keywords might do these avatars some good as well!
Soul of Serra's Realm - Lifelink,
Soul of Equilor - Flash,
Soul of Rath - Menace?