+1/+1 counters are about growth, which is basically infinite
The most unusual thing about the Turtle deck is that it is Rainbow - using all 5 colours of mana (resource), which would typically dilute a deck to being unplayable (MTG Commander decks are at most 3 colour, ordinarily) - but is not only playable, but really good fun, with the turtles plus splinter (black) covering the 5
2017-02-09 Even though Metallic Mimic is a Shapeshifter, other Shapeshifter creatures you control won't get a +1/+1 counter unless you chose Shapeshifter as Metallic Mimic entered the battlefield
Borne Upon a Wind can also be cast when it's your opponent's turn, or even during your upkeep, to potentially steal a win right under their noses
Loxi: Ironically, I think most people will play this for the Cyclonic Rift part of the second bit and just use this as great mill/discard-fodder
Obviously these may be a miss for your format for other reasons (power, archetype mismatch, rarity, don't run planeswalkers etc) but here's a few that I found interesting, divided by broad category rather than color: Actual Modal Dual Faced Cards: While I'm still not done reading Cosima, God of the Voyage (I'm on chapter 2, the language is a bit dense) these cards are ones I do like both sides of
