At this point, I've figured there are enough four CMC ramp cards that you could play a turn two Radha into a turn three 4-mana ramp spell, into a turn four threat. There are 5 sorceries that are rough analogues of Explosive Vegetation, and Overgrowth/Dawn's Reflection/Market Festival/Garruk Wildspeaker/Frontier Siege round out the remaining cards that will add two mana on the next turn to 10, which feels pretty consistent/good. There are definitely other 4-cmc cards that generate mana that could be included, but haven't been (especially Thran Dynamo and Firemind Vessel). Radha fits in as an ultra-reliable 2-mana ramp spell. She also, it turns out, is really good at eating edict effects.
I decided to make my threats mostly dragons, since I've wanted to build a deck in which I could choose either mode of Frontier Siege. The idea is that my curve basically starts at 5, and that each top deck is either good support or a large evasive beater.
So far, the deck hasn't disappointed. Even if Radha dies before I can table a 4-mana ramp spell, the deck usually has something to do on turn three with a few support enchantments, or hate cards.
General
Radha, Heir to Keld
Creature
Territorial Hellkite
Thundermaw Hellkite
Stormbreath Dragon
Glorybringer
Nesting Dragon
Scourge of Valkas
Flameblast Dragon
Hellkite Charger
Hellkite Tyrant
Lathliss, Dragon Queen
Savage Ventmaw
Scourge of the Throne
Atarka, World Render
Balefire Dragon
Knollspine Dragon
Tyrant's Familiar
Bogardan Hellkite
Utvara Hellkite
Xenagos, God of Revels
Bane of Progress
Ghalta, Primal Hunger
Artifact
Sol Ring
Dragon's Hoard
Mirror of the Forebears
Enchantment
Frontier Siege
Dawn's Reflection
Market Festival
Overgrowth
Dragon Tempest
Fires of Yavimaya
Warstorm Surge
Sylvan Library
Crucible of Fire
Sorcery
Skyshroud Claim
Explosive Vegetation
Circuitous Route
Hunting Wilds
Ranger's Path
Rishkar's Expertise
Soul's Majesty
Harmonize
Uncage the Menagerie
Primal Command
Subterranean Tremors
Chain Reaction
Blasphemous Act
Hull Breach
Vandalblast
Mizzium Mortars
Disaster Radius
Instant
Hunter's Insight
Commune with Lava
Abrade
Beast Within
Chaos Warp
Spit Flame
Pit Fight
Planeswalker
Garruk Wildspeaker
Garruk, Primal Hunter
Chandra, Fire Artisan
Sarkhan, Fireblood
Land
Kessig Wolf Run
Homeward Path
Scavenger Grounds
Endless Sands
Command Tower
Rootbound Crag
Temple of Abandon
Sheltered Thicket
Karplusan Forest
Gruul Guildgate
Raging Ravine
Mossfire Valley
Tectonic Edge
12 Forest
13 Mountain
Underperformers:
Uncage the Menagerie, despite looking good in theory as a draw spell, has felt too clunky to work in this deck (the theory being that I could very easily cast it for 5GG drawing five 5-drop creatures, or for 6GG for six 6-drops). This may be because actually drawing 5 midrange/heavy beaters is unmanageable to push them out quickly without more support like Dragonspeaker Shaman. It makes it, somehow, less good than "draw 5".
Dragon Tempest felt like it should have been an all-star, but it was merely acceptable. I rarely had more than a couple of dragons on the field, and I have too few ways to really push out a bunch of dragons, making it one or two damage per trigger. The haste is cute, but, many of the dragons already have haste, and the deck also has Xenagos and Fires of Yavimaya. Fires feels worse, since the +2/+2 rarely matters, but at least Fires never promised more (and really should be Rhythm of the Wild if this deck wasn't just piled together).
Scourge of Valkas felt similar to Dragon Tempest in that it rarely did more than a couple of damage. In the deck's current iteration, Lathliss, Utvara Hellkite and Nesting Dragon are the only creatures that can make more than a couple of extra dragons. The triple-red investment does not help Scourge's case, either; without investing in another Stomping Ground and Cinder Glade, many of the 'forest' ramp spells won't actually push out much more red mana and help cast Scourge. I could include a spare Taiga I have lying around, however.
Spit Flame; my god this card is hot garbage. The mana investment to torch little creatures is way too high to justify the existence of this thing, which I had hoped to play using Radha's RR-while-attacking text. It turns out plenty of dragons have firebreathing, and that mana would never go to waste anyway. The cost efficiency just isn't there, when I could play something like Sarkhan's Unsealing and just torch creatures (AND/OR players!!!) all day every day, even with my non-dragon inclusions.
All-Stars:
Frontier Siege did exactly what I dreamed it would. I ended up choosing Dragons in all the games I tabled it (because I had already Radha'ed into my ramp spells when I drew it), and I got to play giant flying dragons and have them eat my opponents creatures with impunity. It was glorious.
Savage Ventmaw being great when it doesn't die is probably not news to anyone. Its (relatively) tiny body is disappointing, but the mana it generates is so, so silly. The only disheartening aspect is that it's basically post-combat mana. Notable that it combos pretty well with Hellkite Charger, and could potentially go infinite with it and Radha, Heir to Keld during the attack step.
Primal Command was amazing in every game I drew it. Finding a specific and powerful dragon was excellent, and I often slowed down my opponents by putting a costly setup piece on to of their library, or erased their graveyard. I've played the 7-life mode in desperate circumstances before, as well.
Atarka, World Render with haste is a horrifying. Even one other dragon on the table makes her must-answer.
Thoughts on potential changes/upgrades:
I really want Dragon Tempest and Scourge of Valkas to do good work instead of just being token slayers. Because of Scourge's mana cost, that probably means upgrading the mana base with the Taiga I already own, and maybe getting a Cinder Glade or Stomping Grounds in addition for the three search-forest cards I'm running. I've been thinking of including Kindred Summons and Kindred Charge as well; both could really make Scourge or Dragon Tempest shine.
I also strongly appreciate Frontier Siege's persistent fight ability, and it makes me want to try to include Ulvenwald Tracker, Gruul Ragebeast, Arena and possibly Magus of the Arena. Between Scourge of Valkas, Dragon Tempest, Warstorm Surge, Tyrant's Familiar and Frontier Siege, dragons just hammering other creatures into the dirt was a major theme in the games that I won. Sarkhan's Unsealing, despite the deck having (surprisingly) few 7-power creatures, also feels like it deserves a slot for the same reason as the other cards.
Fires of Yavimaya really needs to be Rhythm of the Wild. The dragons that would get redundant haste could have +1/+1. I could never fear a counterspell once it's on the table. Fires is pretty obsolete.
Goreclaw, Terror of Qal Sisma would also be a shoo-in for (vulnerable) ramp plus some utility.
The biggest notable anti-synergy has been Bane of Progress and the powerful enchantments in the deck. I was never happy to have Bane in my hand, and my the friends who I was playing with had specifically wished they had a Bane of their own to play (even to the point of naming it with Conundrum Sphinx) without knowing I had mine.
@bobthefunny Since we were talking about it on Tuesday night and you asked!
Changelog
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July 23, 2019
-1 Mountain
-1 Forest
-1 Bane of Progress
-1 Fires of Yavimaya
-1 Uncage the Menagerie
-1 Disaster Radius
-1 Spit Flame
+1 Taiga
+1 Stomping Ground
+1 Drakuseth, Maw of Flames
+1 Rhythm of the Wild
+1 Kindred Summons
+1 Kindred Charge
+1 Sarkhan's Unsealing
-1 Mountain
-1 Forest
-1 Bane of Progress
-1 Fires of Yavimaya
-1 Uncage the Menagerie
-1 Disaster Radius
-1 Spit Flame
+1 Taiga
+1 Stomping Ground
+1 Drakuseth, Maw of Flames
+1 Rhythm of the Wild
+1 Kindred Summons
+1 Kindred Charge
+1 Sarkhan's Unsealing