These are really great! Here's my card-by-card reaction:
Dawnbringers' Blade - Reading through this card, I really want it to proc on damage to PWs. Nothing other than preference driving that though.
Endless Possibilities - To me, Mythics are also a space to play with unique card text. Imagine the text on this card opening with "Each player reveals his or her library. . . "
Archfiend of Envy - Awesome. This is a card I could easily imagine ripping from a pack.
Charge into Battle - My only criticism is that of flavor. The name doesn't quite describe what's happening. If we're charging into battle, where are the temporary clones coming from?
Marcois Whitvale - His -2 is a little out of place, and kinda weak. That being said, I love his +1. I don't think I've ever seen this variant before. . . too many custom green PWs are +1: rampant growth without a second though, this one creates some great gameplay decisions. It fails the +Loyalty rule in an interesting way in that there are times where you don't want to use it instead of you can't use it. Many Lilianas do it with discarding, so why not with ramp? Super cool and thought-provoking card.
Ausra, Angel of Purity - I would nuke the CMC clause. For one thing, I believe it needs more text to specify that it's among permanents they control. Furthermore, it's already a complex ability that is powerful enough as a [[World Queller]] [[Grave Pact]] hybrid.
Dyra, Sylvan Reaper - I like this card, but I have some worries about the power level of the first ability. It almost seems powerful enough that I'd outright ignore the 2nd ability to take advantage of being able to [[Vindicate]] at instant speed. Adding a Tap to the activation or a 'once per turn' clause would make this card more interactive. I'd want to playtest it though, I could be completely off here.
Brivibas, Realmcrafter - This is a really quirky legend that I'd use as a commander in a heartbeat. What happens when a Planeswalker gets a realm counter? . . . Oh man, now I'm excited. Imagine building your own PW, Karn's +4 into any ultimate you want. I'll take 4 please.
Leudast, Grand Augur - 4 of this one too. I'm the exact demographic for this card and I love everything about it.
Volsung, the Vanquisher - This card is a bit on the nose. I'm not the biggest fan of cards like this at Mythic, because they're usually not a Rare because of how the numbers add up. I usually end up mentally cataloguing cards like this as filler. Not to say it's a bad card, because it's great and spot-on for a Boros beasty.
Vanai, Soul Shepherd - Being a Planeswalker, this card is hard to evaluate. That being said, I'd rate it just beneath 'too strong.' The -3 is good, but not pushed. The ult is so situational it hurts, and is a testament to wow-factor creep. Now, let's talk about that +1. It's beautiful. It's a roundabout way of making a Planeswalker that's combat-oriented. Now instead of gaining you life via Loyalty Points punching bag, she bolsters and says 'bring it.' Can your opponents push through damage? Do you have combat tricks? Do they? This card would be [[Jace, Memory Adept]] tier in limited and format-defining for RDW. But (and here's the best part) it's totally fine. Her +1 folds to [[Dreadbore]], [[Utter End]] and any other non-combat damage PW answer you can think of. Against the wrong deck, her +1 is dead and that makes this card very sub-par. I would love to watch the price of this card from announcement to prerelease and beyond.
Thea Ros - I'll be honest, I'm a lot less excited about this walker. My immediate reaction (and remember, I love UR) is "Why am I paying 6 for this?" The +1 is wow, but at 6 mana it had better be. The -3 is ok, but at about the same level as Vanai's. In other words, Thea costing more than Vanai shouldn't result in such even loyalty to mana ratios; -3: [[Devour Flesh]] vs -3: [[Echoing Truth]] sucks, especially when Vanai's is already over-costed when considering the rest of her card. (Why the 'not named Thea Ros' clause too? Could it cause something degenerate that I'm not seeing?) The ult is incredible and the type of thing that I'd expect on a 6 mana walker, but the top two abilities need some work.
Thanks for sharing! I can't wait to see the rest of the set!