Rakdos Cackler - Goblin Guide is a better card in most situations. Goblin Guide is also about $96 for a playset. Rakdos Cackler, meanwhile, is about $0.48 for a playset. Are you going to block with either creature? Probably not. Is Haste nice? Yes. Is Haste + the potential mana loss worth $95.52 in marginal cost? I don't think so. Out of all the non-Guide one drops, I like Rakdos Cackler the best, but I could be convinced by the right argument that Bellows Lizard, Jackal Pup, Firedrinker Satyr, Goblin Cadets, Tattermunge Maniac, or whatever is better. Just make a good case and I'll consider it.
Ash Zealot - Pretty much auto-include in red decks, yeah? Passes the vanilla test as a 2/2 for 2, has first strike, has haste, has built-in graveyard hate to punish Snapcaster Mage or whatever other graveyard trick your opponent wants to pull. There's no downside. Just an all around great 2-drop.
Boros Reckoner - Another guy that passes the vanilla test at 3/3 for 3, hurts you even if you block him (or if he blocks you), and as long as I have an open mana I can give him first strike to help him stick around for longer. Pretty hard to beat that in a 3-drop.
Solemn Simulacrum - Big Red needs to accelerate mana, but Seething Song is banned in Modern. Red in general needs to draw cards. Sad Robot accomplishes both things.
Fanatic of Mogus - It's a mono-red deck. This guy likes that. 4/2 for 4 is bad on the vanilla test scale, but the effect can easily make up for it in face damage.
Purphoros, God of the Forge - Another guy who likes that it's a mono-red deck. Provides value even as an indestructible enchantment, throwing around face damage every time I summon a creature and giving all my creatures Firebreathing (although at a premium for the ability). Get five red mana symbols in permanents, and he also turns into a big, dumb creature. 6/5 indestructible for 4 mana with those other abilities is pretty stat efficient to say the least.
Inferno Titan - Big dumb creature that 187s lesser creatures (or does some face damage) when it enters the battlefield and whenever it attacks. Oh, and he has firebreathing. He's a win condition all on his own. The epitome of a "big red" card.
Lightning Bolt - Unless things have changed a lot since I last played, this is still an auto-include as a 4-of in red decks. 3 damage for 1 mana is a ton of value - it'll kill a lot of early creatures, and it can also reach for that last bit of face damage you need.
Magma Jet - 2 mana for 2 damage is pretty wimpy considering that Lightning Bolt is a thing, but with red's lack of card draw the Scry 2 is nice. It also combos well well Isochron Scepter (which I'll get to later)
Incinerate - Bigger Lightning Bolt. Probably the better Isochron Scepter target.
Isochron Scepter - All of the Instants in this deck except for Chaos Warp can be attached to this thing, and they all do pretty well as repeatable effects. Lightning Bolt on a stick? Yes please. Magma Jet on a stick? Why, yes, I will enjoy a Sensei's Divining Top that does 2 damage per turn. Playing a lot of artifacts? Better hope I don't attach Smash To Smithereens to Isochron Scepter. One of red's weaknesses is that it relies on cheap spells that can only be used once. Isochron Scepter helps stabilize it. Whatever I attach to it, the opponent will have to use removal to deal with it, because allowing it to stick around will not be pleasant.
Ruination - Blood Moon is a great card to run in mono-red decks. Blood Moon is $35 a pop. But here I am, still running mostly basic lands, still wanting to hose people who don't do the same. Ruination is a reasonable substitute. It's a one-shot thing as opposed to a lasting effect, and it doesn't hose fetch lands like Blood Moon does, but it can still wreck a mana base.
Reforge the Soul - The closest thing red has to good card draw in Modern. Since we're running Magma Jet, there's a chance we can set it up for the Miracle cost. It's not a great card, but it beats Browbeat, and card draw is a known weakness of mono-red decks.
Koth of the Hammer - Can do so many things for this deck. We're running 17 mountains, and any one of them can become a 4/4 creature every turn. He can accelerate our mana to throw down a big threat. And if you can get his emblem off, every Mountain can ping your opponent or their creatures. Just too versatile not to include.
Valakut, the Molten Pinnacle - It might be rare to actually see the ability of this land, but there's really no downside to playing it, either.
Thespian's Stage - Can't find Ruination? Copy your opponent's land.
Ghost Quarter - Non-basic land hate.

[reply]
Ruination isn't legal, but maybe you realized this because it's not on the decklist.
You're gonna need more than 20 land if your mana curve is that high. Also, it seems a bit awkward to have a curve that sparsely distributed. You have only 4 one-drops, 4 two-drops, and 4 three-drops. Either you should lower the overall curve to make it easier to hit creatures on turns 1-2-3 (the aggro plan), or just do what Skred Red does and use burn spells/[[Pyroclasm]]/[[Volcanic Fallout]] in the early game and just skip to 4-5-6. (Keep Reckoner though, that guy is great on defense and combos with Pyroclasms.)
Taking a page from Skred Red, if you go the controlling route, you can also try cards like [[Mind Stone]] and [[Relic of Progenitus]]. Mind Stone accelerates you, and Relic lets you disrupt annoying [[Tarmogoyf]]s and [[Snapcaster Mage]]s. Both of them can be cashed in for a card if they aren't needed.
Reforge the Soul is pretty bad. You don't really want to give the opponent a full hand.
Good cheap sideboard cards: [[Boil]] for blue decks, [[Rending Volley]]/[[Combust]] for Splinter Twin, [[Molten Rain]] for Tron and multicolor control/midrange, [[Shatterstorm]]/[[Shattering Spree]]/[[Vandalblast]] for Affinity, [[Dragon's Claw]] for burn, [[Pyrite Spellbomb]] for [[Kor Firewalker]]. [[Ratchet Bomb]] is a nice multi-purpose card as well.