My initial feeling immediately is -1 Return, +1 Charioteer. Mass trample removes chumping just as well as 2 global damage, except Charioteers turn sideways.
Mina and Denn feel expensive and slow. Also, you're lacking Voltaic Brawlers. I don't think the Spire synergy is worth it. Side note; change Spires to Hubs if you make this change, probably.
Avacyn's Judgment should probably be a 1-of. Makes room for the 3rd Voltaic. You can probably get by with 1 Manticore, and I think the Gearhulk is very expensive for what it does; you're generally hoping it increases your damage by about 4 that turn, possibly more later. For that same 5 mana, you can cast
Insult // Injury's first half and Fling. That will probably do more than 4 immediately.
I haven't experienced Hazoret yet, but the Incarnates are probably great with mass trample. If we allow for all the changes proposed, I've left you a free slot by dropping a Manticore to experiment - or use the 2nd Insult with. Outland is probably very much worth testing with Fling. Outland would also make good partners with
Samut, Voice of Dissent if Hazoret doesn't work out.