Much like the Allied color thread, these cards were designed to push 2-Headed-Giant players toward themes of the related colors. Relevant themes are listed in a spoiler below.
Enchantment - Aura (R)
If damage would be dealt to you, it is dealt to enchanted creature instead.
Copy target insant or sorcery spell twice. You may choose new targets for the copies.
Creature - Beast (R)
Whenever a player on your team cycles a card, that player may put a +1/+1 counter on target creature and regenerate it. (You always choose the target.)
Creature - Human Soldier (R)
Whenever Povrix Elite attacks, put a +1/+1 counter on each attacking creature. 2/2 1/2
Creature - Merfolk Shaman (R)
Each player draws an additional card during his or her draw step.
Whenever a player on your team draws a land card, that player may reveal it
and put it onto the battlefield.
Why is hybrid in this set at all? We were trying so hard to make players careful about what colors they draft (hence one reason for sidekick being off-color) in order to add depth to the environment. A mono-blue card is still probably going to see play in somebody's deck. 4 of the 5 colors are probably being played, maybe splashing the 5th, between the two decks. What's the point of having hybrid? It's added complexity, so how does it support the design goal? I don't get it.
I'm designing some cards as hybrid cards that can go in either color, then based on feedback and what other slots get filled putting them in a monocolor rare slot. Sorry if the use of hybrid caused some confusion or distress.
Twin Echoes - Wild Ricochet says this is probably undercosted, factoring in hybrid mana, due to untargeted cards such as Divination that this can hit. Then again, this is supplemental, so it could be fine. Playtesting is key here.
Writhmonger - If you always choose the target, why not just say "Whenever a player cycles a card, put a +1/+1 counter on target creature and regenerate it."? It's a lot less confusing and doesn't need reminder text...
Povrix Elite - It may be fine, but it seems very undercosted to me - its a 3/3 first strike for 3 (with hybrid upside) that pumps your whole line. See Herald of War or Fangren Firstborn, which I'd say are both quite weaker than this card.
For what it is worth, a few hybrid cards seems okay. If both players on a team are playing at least one of the colors of a strong hybrid card, they can draft the card and decide later which deck to put it in.
Twin Echoes - Strong but fairly simple so it doesn't take up too much complexity points. Probably a little better than Wild Ricochet, especially when you can coordinate with a good spell from your teammate to copy.
Writhmonger - How much cycling is in the set... and how much cycling do you expect late game after you have finally cast this card? What if it just triggered whenever a player discarded a card to broaden the interactions?
Seaweed Oracle - This wording wouldn't work. Miracle has rules that allow you to reveal the card so it can trigger. It would need to be worded something like Rowen. "You may reveal each card you draw. Whenever you reveal a land card this way, ..."