Because you can top deck a win off an empty board more often with R. You get board wiped by a sweeper and top decking a HotP could lose you the game. Not to say it doesn't have a virtue but it needs to be as impactful in a good board state as a bad one.
What's your meta like to want Collective Definance over one of the other removal suites? I don't like pushing 3cmc or higher with out a clear reason. Soul-Scar Mage means you could be casting Volcanic Fallout and cleaning up very well without costing a lot and at Sorcery speed. I'd also like to see one Grim Lavamancer or such as a rescuing "non-combat damage" source for Soul-Scar.
I haven't tested Emerge in a long time and I assume it's a hedge against Fatal Push and Death's Shadow? In that case would it be better moved to the sideboard?
I'd also exchange the 2 Prairies for basic Plains for when you side in those Moons.
I'm bringing my deck with me to work today. New kid at work used to play Magic so I'm gonna check out what he has a d so home some of my stuff. I'll take a run through of my list and try to figure out a good suggestion. It's been so long since I've played now. The meta has shifted a few times since I last played and before that so... But I'm leaning towards suggesting reducing the Soulfure's to 1-2, add Soul-Scars up to 3-4, maybe try some Abbot of Keral Keeps since your curve ATM is low and consistent. Otherwise you could have a Bedlam Reveler or two to reload in the late game and give a larger Fatal Push resistant body. I'm personally a sucker for Prowess. I will freely admit that but it is also an ability that can help throw off opponents' combat math when you surprise drop even a single instant. I believe that was the primary use when I was testing Emerge a couple years ago.
I'm personally eager to start playing again and I want to test both the Ramunap Ruins and the W Desert that can pump your team... At the end of my regular playing I was testing everything I could trying to find a reliable advantage to press in those last points of damage or help scale up my creatures. Kelvin Monoliths could have been great math wise but just never performed. I always wish Barbarian Ring was legal for Modern and Ruins might be that "more balanced" version of it.
Planeswalkers Uniqueness rule will be gone at the release of Ixilan. All Walkers are now Legendary and follow the Legendary rules. Kytheon is now better because you can play him out and still be able to play the other Gideons as when he flips you don't have to pick "do I keep flipped Kytheon or my other Gideon?". We may see a heavy shift toward control in the meta as well as the Boros archetypes may start to merge some more again as you can take that "tribal Gideon" deck and the Boros Pillowfort and literally any other Boros list and mash them together.
OH! And for anyone running Blood Moon or Magus... rules change incoming to be ready for. Lands entering under Blood Moon enter untapped as a Basic Mountain. No Vesuva or other tricks. Outside of Modern this does affect cards like Dark Depths so one could play Moon, play Depths, ditch their own Moon and have a 20/20 death stick. Be aware of the new rule. I have not explored yet if this can be abused with other Modern legal effects yet.