Thundersaur is a giant T-Rex, and that's enough for me to want him. But it also turns out that he's criminally underrated currently, mainly through underexposure. Unlike some of the other powerful new cards (Tarsus Deathweaver, in particular) he is legendary, so you are incredibly unlikely to see a full set of him. As a result the meta doesn't appreciate his value like it does, say, Justicar or Tarsus.
It should be fairly clear that Thundersaur works best with some of the other walls. Heart Tree has always been a strong card, but now has a worthy target to shade. Mossbeard Patriarch also makes Thundersaur grow fast and hard. He also has some great options to be threatening with a simple Battle Techtician or Weirwood Patriarch. My previous wall deck is terribly outdated, and now has 30 worthy cards to play.
The most hilarious synergy though is with Rageborn Hellion. Both benefit from the same wall deck staples, both grow out of control incredibly quickly, and are very hard to take down. But the synergy overflow comes from Thundersaur having Breakthrough, which results in Rageborn Hellion casually getting larger without serious planning from opponent.
Core: Thundersaur, Rageborn Hellion, Heart Tree, Mossbeard Patriarch
With at least 8 health at Level 1 it will be taxing on opponent to remove any piece of the Wall puzzle. Punish opponents for not playing things in front of Thundersaur with timely Hellion plays, protect Mossbeard and Trees to keep the infrastructure in place. Once you get all the lanes filled the game can quickly get out of hand with two growers and lots of buff spells.
Synergy: Weirwood Patriarch, Ferocious Roar, Iceborn Fortitude
Weirwood is a great 7 health, and quickly takes any hard to deal with 8-health walls to 10-health big booty monsters. Ferocious Roar doesn't need it's praises sung, but turns out it's good when you often have full boards and counteracts the Epidemics you'll surely see. Iceborn Fortitude isn't an inherently strong card, but has great value when played with a full board and is at it's strongest in this deck.
Support: Ashurian Mystic, Global Removal, ??
With the remaining 9 cards we have to remember that they won't be played very often. Our core is going to be leveled aggressively, and our synergy cards are going to be better in almost every situation. That means that our last 9 cards will have to be situational, but very valuable those situations. A variance in answers is more likely to be important than any particular card. Without testing it's hard to give any sort of numbers.
Ashurian Mystic has strong synergistic value. His primary role is to simultaneously grow a Rageborn Hellion or two, while taking pressure off the Treelords. However, he likely only sees play when immediately followed up with Ferocious Roar or Weirwood. Luckily for him his aggressive attribute is still valuable late game.
While we don't necessarily want to be leveling removal aggressively, when we do need removal global removal seems more appropriate. We have choices. Firestorm has good synergy with Heart Tree, Noxious Cloud is slow but is guaranteed to be effective. Pyre Song is interesting since we are actively trying to fill lanes. But, as a general rule, Thundersaur in front of major threats is the best answer. For example Level 3 Zimus just gets completely walled by Thundersaur.
Tossing in some high power threats is also definitely worth consideration. Storm Bringer might be able to tank and regen up any battle damage, and Swampmoss Lurker also does 8 damage but scales better (and becomes huge with Iceborn Fortitude!). But, I'm leaning towards Everflame Pheonix for this role. If you have to throw 7 power in front of something may well do it with some of the most powerful bombs in the game. Pheonix Egg + Weirwood wins games, yo.
Soothing Radiance? Probably not, but there isn't a card that breaks trades more than this one, despite how dreadful it is.
And I still want my Chrogias. 0/15 defensive wall too stronk.
PS WHEN I WROTE THIS I COMPLETELY FORGOT ABOUT URANTI FIREFIRST. NOW IT'S A REAL DECK
My IGN is pwndnoob, thank you for reading. Always appreciate comments and questions.