one of the things I liked best about when I was playing the world of darkness games was the way that willpower worked. the basic idea was that your character had a virtue and a vice (based off the seven deadly sins, naturally), and if you wanted to resist your sin you had to spend willpower, usually, and if you acted according to your 'better nature' you'd get some willpower back. you also got willpower back for indulging in your sin, I believe. interestingly, you could also spend willpower to get like, better dice rolls.
ok so, world of darkness games. vampire, werewolf, ghosts, mages, they all have willpower. a character's player can choose to spend willpower to get a boost on dice rolls, or fuel magic, or whatever.
also on a character's sheet are one of the seven deadly sins- sloth, lust, greed, whatever. the character can gain willpower for leaning into their sin, for playing into their darker nature, for showing weakness. the character also has to either spend willpower or take a chance of slipping into bad habits when the target of their weakness comes up. the lustful vampire has a hard time not spending the arms in his or her lover, even when they really should be safe in their stronghold hiding from their hunters... the envious mage has a hard time resisting the opportunity to show off even when they really shouldn't, and etc.
this does a couple of things- it hooks the character and the player together in a really strong way, by making the player care about what the character cares about. if you take care of your in-game avatar, that avatar is more competent, more resilient, and more able to effectively carry out the player's interests in the game. and it also points the GM towards a conflict that the player probably finds interesting, or at least a conflict that the player has already greenlit. a lot of players don't care to have their characters' father or sister as hostages or whatever, but they've definitely given the GM permission to tempt their character with lots of gold, or give them opportunities to showboat and brag, right? and it also lets the player look for these opportunities to let the somewhat more negative side of their character show through if they've had a rough couple of episodes. and that's interesting! there are always lots of opportunities for player characters to be noble and competent, for them to be clean and respectable. 
now that I think about it, though, the world of darkness games are necessarily rooted in a very specific sort of western ethos, that subconsciously christian worldview where pleasures are to be enjoyed in moderation, where you are supposed to congratulate yourself for, essentially, being sober and productive at work and whatever else. that doesn't really apply to the sorts of fantasy worlds I want to emulate. I mean, you can do whatever you want in your world- you can just decide that the seven deadly sins are perfectly acceptable to you. but I don't know- a dwarf is not going to understand that she should limit her gold hoard's size, and elves are notoriously indifferent to the human concept of 'vanity,' let alone pride. halflings don't hold with all that running about nonsense and if you call them 'slothful' you're really missing out on a good understanding of their culture and personality.
but here's kind of more of what I was thinking- make it more personal. I'd have my players consider and detail a particular failing of their character, something that they really struggle with. and they can just be the sins, too! something along the lines of 'my barbarian has a weakness for violence, the more indiscriminate the better,' or maybe a more subtle issue, something like 'my cleric looks down upon those without divine powers and has to remind herself not to treat other as beneath her just because they haven't been chosen by the gods.'
personally, I'd take this opportunity to do some world-building too, and give examples of a couple of cultural folk heroes, demigods, and local celebrities, as well as what makes them such exceptional people. maybe a given dwarf king is legendary due to his wisdom, dispensal of justice, and (naturally) his unbeatable wealth, whereas a dwarf wretch is irrational and poverty-stricken and physically weak, unable to defend his stronghold and despised by his family. maybe this particular band of humans idolizes a cleric-beggar-wanderer who used her powers to tend to the sick and downtrodden and gave the ultimate sacrifice and died to save the world from a very particular evil. and then maybe those same humans look down upon a harsh-tongued, impious ruler who frequents the red light district.
and so if I were going to put something on the character sheet for a dwarf character, I'd go to the part where I describe my character's personality I'd write FLAW in big letters and then write a quick sentence about how my dwarf is arbitrary and occasionally downright cruel, and he struggles with his sharp tongue. and then I'd write some ways in which he's pretty alright, actually. and then by codifying it and making it a big part of my character, by getting an actual bonus for when my character acts up, I make it more than an interesting facet I'm going to ignore in favor of doing what I wanted to do anyways. it goes from an interesting quirk of my character to something that I'm going to invoke at the table, something that I want to have come up. something I'll bring up, if the GM doesn't, because I want to restore my willpower and that's that.
as far as actual willpower mechanics go, I'd do something like the following:
assuming you're playing some sort of dnd adjacent game, you can spend a willpower point at any time to add the results of a rolled d6 to any d20 roll. you can't use willpower to increase your damage or healing or anything else that you'd use any sort of smaller dice for.
willpower is measured from 1-10. characters start with 5 willpower.
if an opportunity to indulge in your flaw comes up and you take it and make things more complicated for yourself and your party, the GM will give you some willpower.
if you decide you do not want to indulge in this opportunity, you'll need to roll for it. roll 1d20. the DC is equal to twice your missing willpower. you don't get a willpower point for this.
you can also decide not to chance it on a roll, and spend a willpower point to automatically succeed at the willpower roll.
if you're using my rest mechanics from earlier , any time the characters have a rest worth at least 110% experience bonus, I'd give them a point of willpower as well. a good nights' sleep does wonders for the human (or dwarvish, or elvish, etc) mind.
if a character ever runs out of willpower entirely, then they immediately start acting out their flaw, gaining a willpower point. this can make for some pretty erratic behavior, but that's the point!
an example: a mighty Barbarian has 3 willpower and her flaw is classic for barbarians: lust. so she's in town and she sees a particularly handsome guardsman with really striking eyes- her weakness. she really doesn't have time to introduce herself... the GM decides to force the issue. the Barbarian's player decides that she meant it when she said she didn't have time. since her willpower is sitting at 3, she's missing 7 points, and the DC is 14. not great odds, but she tries it anyways. if she rolls below, then her character's going to go introduce herself to the guardsman, with the intent of a quick liason somewhere discreet. if she rolls higher, then she's on track (ish) and she can maybe keep the man in mind for another time. if the player had chosen to spend willpower, then she would have automatically resisted the temptation.
it's that simple, really. I'm personally looking forwards to hacking my favorite edition of dnd to include this sort of thing, given that it's relatively self-contained and easy to 'borrow.'
 I know that some people will choose to 'chew the scenery,' so to speak, even when there's no real benefit to doing so. They just enjoy playing a coward who's making bad choices in-game to be more true to their conception of the character. but honestly, it doesn't really work in your stereotypical dungeon fantasy game. there's no reason to be cowardly when you have a full stack of hit points, no reason at all to seduce bartenders or guardspeople or whatever, you know? it can be fun for its own sake but fun is fleeting and when you're done, you basically have a silly anecdote that stands by itself.
 if you're not, I'd give them a point of willpower every other extended rest, or if they pay for good lodgings in a city or something. the whole point of willpower being presented like this is that it is, by design, flexible to the style of play that you and your players are actually participating in. if you have a player who hardly uses the willpower mechanic, that's fine. they're weaker but their experience in the game is going to be a lot smoother. they'll probably get topped up pretty quick and then, since their willpower is high, they won't have a hard time getting