MTG Salvation

Time Spiral or Zendikar would have to be my favorite.

I love the theme of Time Spiral, and some of my favorite mechanics (Suspend, Grandeur, Flanking, Shadow, Kicker, Transmute, Scry, Fateseal, Delve, Typecycling, Fortify, Absorb, Aura Swap, and Indestructible) appeared in it. Planar Chaos's Colorshifting and Time Spiral's nostalgia were really interesting to me.

Zendikar was amazing. Level Up, Totem Armor, Landfall, Traps, Kicker, Rebound, and Quests were all great. Multikicker is my favorite mechanic of all time. The only misses for me were Annihilator, which should have only triggered when it dealt combat damage to you, and Allies, which I think were a horrible try at make Magic more like D&D.

Radiant Zealot WB
Creature-Angel
You can't cast Radiant Zealot during the first 6 turns of the game
Flying, Haste
Sacrifice a creature, reveal Radiant Zealot from your hand. You may cast Radiant Zealot as though it was one turn earlier.
3/3
Radiant's angels tore apart Serra's Realm, slaughtering the plane's inhabitants and sending their souls to Phyrexia.
(This card is a combined version of Fallen Angel and Serra Avenger, representing Serra Angels corrupted by Phyrexia)
Next: A timehifted Radiant, Archangel

Khalcan Tamar, Lord of the Sands 4WRG
Legendary Creature-Sand Warrior Mythic Rare
Other Sand Warrior creatures that you control get +1/+1
Grandeur-Discard another card named Khalcan Tamar, Lord of the Sands. Put 6 1/1 red, white, and green 1/1 Sand Warrior creature tokens onto the battlefield under your control.
6/6

Next, an alternate reality Nicol Bolas, Planeswalker

Graveborn Reaper 3BB
Creature-Zombie
Storm
When Graveborn Reaper is put into a graveyard from anywhere, exile it. You may return target creature card from your graveyard to your hand.
3/1

Next: Timeshifted or Alternate Reality Lightning Storm

Reaper of the Unborn 3BBB
Creature-Demon Uncommon
Flying
Baneful 3 (If you would draw a card, you may instead exile the top 3 cards of your library. You may choose a creature card from among them and put X +1/+1 counters on Reaper of the Unborn, where X is that card's power)
5/5

Next: Flashback+Buyback

Thoughtsilencer of Uthun+Progenitus

Progenitus, God of Destruction WUBRGWUBRG
Legendary Creature-Hydra Avatar God Mythic Rare
Protection from Everything
When Progenitus, God of Destruction comes into play, you may pay WUBRG. If you do, destroy all creatures your opponents control. If you don't, sacrifice all permanents other than Progenitus, God of Destruction.
If Progenitus, God of Destruction, would be put into a graveyard from anywhere, shuffle your graveyard into your library
10/10

I think that you should have at least a red kraken since Tidesprite is partially red and that if you have 4 krakens, it would make sense to round out the cycle and make a white one. Here are some ideas.

Kalimaras, the Ur-Kraken WUBRGWUBRGWUBRG
Legendary Creature-Kraken (M)
Kalimaras, the Ur-Kraken cannot be countered
Indestructible, Hexproof, Swallow, Trample
Whenever Kalimaras, the Ur-Kraken deals combat damage to a player, you control that player during their next turn.
If Kalimaras, the Ur-Kraken would be put into a graveyard from anywhere, reveal Kaimaras, the Ur-Kraken and shuffle your graveyard into your library.
14/14

Zavek 8RRRRR
Legendary Creature-Kraken (M)
Zavek can't be countered
Swallow, Protection from multicolored
Whenever Zavek enters the battlefield or attacks, you may destroy up to three nonenchantment permanents
If Zavek would be put into your graveyard from anywhere, reveal Zavek and shuffle it into your library instead
11/11

Akhanor 8WWWWW
Legendary Creature-Kraken (M)
Akhanor can't be countered
Swallow, Protection from converted mana cost 4 or greater
Whenever Akhanor enters the battlefield or attacks, you may destroy all other creatures
If Akhanor would be put into your graveyard from anywhere, reveal Akhanor and shuffle it into your library instead.
11/11

I also think that all of the krakens should have a mana cost of 8 colorless and 5 colored, and be 11/11. Because of this, I think that Nolgul in Iamur should have 1 less toughness, 1 less power, and cost one blue mana less. Naandr should have 2 less power, 2 less toughness, should cost 2 green mana more, and should draw 4 cards when she enters the battlefield or attacks instead.

City-Devouring Dragon 3BR
Creature-Dragon Rare
Flying
Consume (You may sacrifice any number of permanents to reduce City-Devouring Dragon's mana cost by one mana of any color. If you do, city devouring dragon enters the battlefield with a +1/+1 counter on it for each creature sacrificed this way)
4/4

Ukor, King of the Wastes 3BR
Legendary Creature-Ogre Warrior
First Strike
Whenever a creature you control is put into the graveyard from the battlefield, it deals damage equal to its power to target creature or player.
RB All creatures you control get +1/+0 until end of turn
6/1

Next: An avatar lord

Wurm's Tooth+Lu Bu, Hellspawn Abomination+Zhuge Liang, Shu Prime Minister

Gatharek the Wurmslayer 3UBBB
Creature-Demon Warrior (M)
Gatharek the Wurmslayer can block creatures with landwalk abilities as though they did not have those abilities
Whenever a player casts a green spell, you may exile any number of cards from your graveyard. If you do, that player loses X life and you gain X life, where X is the number of cards exiled this way.
6/6

Judgement Dominator+The Brute

Brute's Judgement 3BR
Enchantment Rare
BBB: Target player sacrifices a permanent
RRR: Regenerate target creature

Guildmaster Iranitev+Persuassion

Dominating Shadow 4BBB
Creature-Spirit Mythic Rare
Whenever a nontoken creature you control dies, gain control of target creature until the end of your next turn. Untap that creature.
Sacrifice a creature that you control but don't own. Put a token that's a copy of it onto the battlefield.
3/3

Isn't it weird that after 8 posts, we are right back to dominating stuff like Icd's Ronkonkoma?

Grim Herald+Sprout Swarm

Herald of Rebirth 2BG
Creature-Fungus Shaman (R)
Whenever a nontoken creature you control dies, put a 1/1 green saproling token onto the battlefield.
Sacrifice a creature: Add 1 mana of any colors that that creature has or 1 colorless mana to your mana pool.
3/3

Ok, but remember my suggestions for mechanics for later sets in the block. Also, famed counters could be in the same set as +1/+1 counters. Rise of the Eldrazi had both level counters and +1/+1 counters.

I like Nixie too. I read what you wrote about how Mermaids are to Merfolk as Humans are to Elves and I get that, but most other people won't. Nixie would help prevent confusion about that.