I'm looking for cards that make you a lightning rod for your opponents with some benefit where if they don't, they'll suffer.

The poster child for this is Luminarch Ascension; fail to consistently attack the Luminarch player and you'll get absolutely crushed under the weight of 2 mana 4/4s.

Preferably, I want them to take an actual attack step against me. Hit me with your suggestions!

Edit: Not planeswalkers.

Off the top of my head:

Bloodchief Ascension
Sygg, River Cutthroat
Dissipation Field (doesn't have a benefit, but guarantees that every random solemn and wood elves will come at you if you look open).
Bitter Feud (for that player)
War's Toll
Exquisite Blood

On a more general note, just play strong non-creature permanents and don't have blockers.

In multiplayer it's hard to force opponents to attack you instead of mutual opponents (unless you control them), or punish them for not doing so. But I found Alluring Siren, Taunt, and Trove of Temptation.

You can reward them with the C13 and C17 curse cycles. C17 even has some upside if you enchant yourself. Forcing all creatures to attack may work if you're the safest target, but presumably you're also planning to punish them for attacking you, rather than just trying to die as fast as possible.

The backdrop to this is that I built O-Kagachi, Vengeful Kami, and I'm finding that people just don't want to attack me. And, like, this is fine for the most part, but they'll eventually answer O-Kagachi, and the only thing I've gained is life total (that I haven't lost from attacks).

I want to punish players for not attacking me (and subsequently getting some sweet vengeance). If they're not going to attack me, I want more than just a higher life total. Basically I want a double bind where you either attack me and let me punish you with O-Kagachi, or you don't attack me and I will punish you with X (like Luminarch Ascension).

I also don't want to just blanket provide incentives to attack me without getting much out of it myself (like playing a Curse of some kind of myself). I want cards whose benefits hinge on not being attacked.

Trove of Temptation, Coveted Jewel, and Luminarch Ascension. Alluring Siren is also in, but I haven't had much luck with it yet.

I also play Maze's End, which kind of punishes players who don't attack me or don't have an answer; I just kind of eventually win if they don't consistently come knocking, but it's so very slow. I also plan on putting some Monarch cards in.

It's more of a behavioral thing than actual card mechanics, but Rhystic Study and other noisy annoyance cards are good for encouraging folks to come at you. Same for camping on repeatable aggravators like Capsize.

Interesting points. My group is... interesting that way; few people play them, and it they don't really provoke attacks. Players are usually fairly spot on with their threat assessment (which is why I'm looking for mechanical double binds).

This was my first thought - being the Monarch is a great way to encourage people to attack you, and rewards you with card advantage if they fail to do so. It's also pretty relevant at any point in the game, can't be removed once it's been introduced to the game, and your general is good at reclaiming it once it's been taken from you. Plus it plays off people's greed.

Luminarch Ascension is a thing, but not one I'm fond of. My experience with the card generally goes one of two ways. A: It drops late-game and gets removed almost immediately, or B: It drops turn 2, goes active partway through the third turn cycle, and the game ends way too fast under a tide of angels. I've almost never had a middle-ground situation where it wasn't removed but also didn't take over the game.

So this is a bit antisocial and a generally weird suggestion, but have you considered just being more annoying? Your group may be cold-hearted, calculating veterans, but at some point even the most placid Ferdinand is gonna get upset if you poke him with enough bee stings.

A handy way to goad players into action is with value-adding cantrips. Cards like Smash or Disrupt are good ways to get people moving, especially when you go after their early game mana plays (squash a Sol Ring, counter a Cultivate). Most players avoid running these 'cause they'd rather have a bigger effect, but if you play them early, you're really just cycling cards on turns when you wouldn't be doing much otherwise. Similar with effects like Blind Obedience; they're easy early plays that don't cost you much, but still provide an obnoxious speed bump for everyone else.

No, they're not a mechanical lock (frankly, there aren't many of them), but much like playing a stax deck, you're goal isn't really to put a hard lock on people... your endgame is to bleed so many resources off your opponents that they die or give up.

I'm more interested in having fun, than winning. Your suggestion is well taken, but, I think it's not for me. In a competitive game, I would absolutely be down for the mindgames, but, I like these people, haha

While our group isn't competitive (we're very laid back, and generally shy away from combo, tend to play soft wincons, etc.) this is something we do. Even value-neutral cards are welcome, if they have potential later (Vandalblast, for example, is highly played in our group).

This is definitely the angle I'm looking for. Though, I'm finding more and more, simply being a cantrip isn't really enough justification to play a card. I find myself favouring charms because their versatility usually makes them playable and that minor effects if used wisely can be powerful (currently in my list is Sultai Charm and Abzan Charm, both of which at least have outs for just drawing cards).