I'd like to take a second to endorse a few cards as either underplayed or undervalued in modern. This isn't necessarily an endorsement to play these cards over the titans of the format like Ragavan, Grief, or Murktide, but simply a discussion on undervalued assets that can round out existing strategies and work to formulate new ones.
Some shorthand arguments will be helpful, as they apply to more than one card.
BOGO- buy one, get one. The idea here being, you get at least 1.1 material per card, and when stacked, leads to material advantage that can act as a win condition via inevitability. Notably, BOGO mainly works through adventure cards, the primary hallmark of which is being ok in the opening hand and good in the middle game because they can be half played then stored for later turns, and phenomenal topdecks, as they can be played to full capacity immediately.
Bonecrusher Giant- BOGO, quick damage that matches perfectly with many weenie threats in modern (Ragavan, Grief, Hammertime critters). Also, a 4/3 three-drop that's almost guaranteed to go one for one with a shock damage on top and a five turn clock ain't nothin to fuck with.
Heartflame Duelist- BOGO, applications in burn that could potentially shift the entire strategy, as you can stay alive longer and thus throw more burn at opponent before bighting the dust. The 3 damage three-drop instant isn't great, as it's a two-mana worse Lightning Bolt, but with the BOGO aspect and the potential lifegain, it's worth considering.
Hearth Elemental- I have no god damn idea whatsoever why this card isn't being abused like there's no tomorrow. Used in a deck built around it, this card becomes a three drop 4/5 with an 'etb, draw 2 cards'. Unbelievable BOGO applications, and could be splashed into soooooooo many existing decks. I particularly think it would go well in Murktide builds.
Kellan- This card is just stupid when you realize it's a literal Open the Armory with a creature welded on top. That's just nonsensical to me, but it exists. The applications are obvious. BOGO like crazy, as it can fetch equipments or an O Ring to deal with a problem. It can even be further abused by fetching reconfigure creatures or Spreading Seas to nerf a land (hello Tron and Titans). But even on top of this, three-drop double strike with a lord ability that fetches it's own nonsense boosts has plenty of potential. Hell, you could fetch an oldschool Bonesplitter and for 2 mana your three-drop is swinging for 8 damage and your tokens are all +1/+0.
Mosswood Dreadknight- like, why has this card not caught on yet? If you kill it but don't exile it, you're helping opponent. You turn a Lightning Bolt/Fatal Push/Unholy Heat/so many other cards into a 'opponent loses 1 life, draws 1 card, and later gets another 3/2 Trample'. That's a -2 interaction that nets you (the bolt player) 1 life. BOGO baby, BOGO. I legit think decks should be built around this champion of the BOGO ethos.
Obsidian Charmaw- not supremely useful maindecked, but a cheap alternative to Blood Moon as a side against Tron that smacks the deck cold with a declaw and a 4/4 Flyer to deal with that's immune from 75% of the popular removal run in modern. Opponent will probably have 2 colorless lands at least in the opening, so he will probably cost 3 or less and will cut opponent off at the knees when against Tron.
Ogre-Head Helm- BOGO applications out the ass. Fetchable, reusable, draws you three cards on its own or while eating equip targets. It's reconfigure cost is high, which is lame, but in reality you won't be using it that way unless it's a topdeck, in which case, you can drop it, reconfigure, swing, and hopefully toss your non-hand and pick up 3 cards all in the turn.
Reinforced Ronin- What's not to love about this Goblin Guide that doesn't help opponent and helpfully bounces itself when its not useful anymore so it can be cycled off for something you need? God damn, pair this boy with Ragavan and smash faces.
Scalding Viper- the little brother of Eidolon of the Great Revel, except it only hurts opponent, nets you a tempo, and probably nets you a free ping from that tempo. If this card survives, it's easily worth 5 damage all on its own, while also giving the BOGO bump in tempo.
Shepherd of the Flock- a blink-ish one-drop that later turns into a 3/1 two-drop clock or chump blocker. Helps reuse ETB effects, save critters from removal, block shenanigans, landfall, etc. I don't think it's worth running over other staples, but could be a fun addition to Death & Taxes type decks. Notably also works with Grief.
Damn- why are all black/white builds not running a copy of this? I can think of no logical reason. Unqualified two-drop critter hate with an attached nuke!?!?!?! Ya, it's a sorcery, but who fucking cares.
Hard Evidence- I've been going to bat for this card for a few years now. It's the original BOGO before the adventure cards hit the scene. Blue is notoriously awful at protecting itself on the field, and this is a one-drop 0/3 wall that stores a draw you can pop next turn or whenever. What's not to like about a turn one answer to Ragavan that's also a stored cantrip?
Electrolyze- extremely versatile cantrip shenanigans. I love throwing in two copies in a deck and topdecking one when I've got five lands available and either sniping a threat with tempo and material or just shocking opponent and being that much closer to the W without really losing anything.
Kaya's Guile- this one sees play, but not nearly enough by my estimation. Mainboard grave hate, big nasty critter hate, life if needed, flying token kicker. Basically never a dead draw, as it helps you out with most situations common in modern, and worst case it's a 1/1 Flying token and 4 life at tempo. Mainboarded instant grave hate!!!!!!
Tyrant's Scorn- the main draw I see with this one is that it helps screw up the big bads that only work once, like Murktide, or get reanimated and are hard to combo back in again. It's not going to save you against Grisilbrand or anything, but is a solid side option as it also handles the majority of critter threats at tempo and is an absolute smash to the face of the Murktide player.
Cycling lands- I've got a whole theory on these boys. Long story short, I now run 24 lands in the average deck. 21 normal build and 3 cycling one-drop lands that are really just mulligan banishers that are great topdecks, aren't bad in the opening hand, and really only help more than they hurt.
I'm open to discussion about any of the above, the concept in general, or any ideas for further submissions. Feedback appreciated