This stems off of another thread on here but I don't want to hijack it, as this is more of a "what if?" than a "This is how I do it".

So Lycanthropy is one of those curses which I intend to make somewhat unique mechanically in my world, and I have a few ideas for this:

The premise is that the lycanthrope is another entity who possesses the body of the afflicted - that's how they reproduce, it's by infacting people. When the moon is full, they take full control of the body for the night, so they become a werewhatever, but I am considering ways to work with the idea of this being more of an issue than a once-a-month thing, and of making it a tempted-to-keep sort of thing where it's not just down to "do I want to play a werewolf". It can become a personal journey for any PC (or NPC) who contracts it.

My idea is that the transformation can also occur when a PC drops to 0hp. If the PC fails a death save, then they start to transform. If they fail two, then they complete the transformation and become a werewhat under DM control, with some amount of temporary HP (probably level related, I imagine powerful heroes with were-curses to be more powerful than frank the suspicious commoner), who's sole concern is self preservation - so that means they're running off and eating something.

This could mean that, whilst you've foreshadowed it ahead of time, the players' first realisation of the were-curse is their character starting to turn during combat, then running off.

The idea is that, after a short amount of time, the were-curse will have allowed the PC to recover 1hp, and emerge alive but with, say, 1 or 2 levels of exhaustion and a complete blank as to how they ended up in the woods surrounded by dead sheep. I don't want this to be something which will stop someone playing for too long!

Then they have to decide - whilst they're a werewolf, they get a second chance if they die, which is a seriously good boon. But, they will also lose control and cause havoc in the world, and may be hunted down, which is very bad.

I might also include a mechanic where it starts taking over when they have HP left, and the amount increases each time, if they choose to embrace it.

It's a half-baked idea right now, but I wonder if it would make lycanthropy an effective plot device?

With a group of players that’s bought in on your campaign, anything is possible.

Do I think it would be good as a plot device? If the player wants to get rid of it, sure.

If they don’t, and they don’t become an npc after the full change, you’re basically handing out free feats for nothing. Claws, bite attack, damage immunity unless it’s silvered or magic, and the keen hearing and smell wisdom rolls with advantage. 

However, you’re mixing your metaphors in your post, at one point you say the lycanthropy is another being that takes over the characters body and then you say the character is in control, I figure you’re just writing to show the ideas as fast as possible and you didn’t notice that. 

either way, I don’t agree with using it because its a potentially unbalancing boon to a single character. 
Your miles may vary.

Not sure where I said that player has control, but the idea was that no player would have control over their were-form. At first it would be like a semi-good thing, where if the PC dies they instead go on a rampage as a were-thing and live, likely just described by the DM as the aftermath - which NPC's might guess, who's died, that sort of thing.

The later part where they embrace it - this meant for if the players decide "nah, I don't want a cure, I want to stay with this feature", it would get worse, starting when they have 5hp left, or 10, or 15, or if they take X damage in one round. The Were-creature will find it easier and easier to take hold, but each time it does, it's back to DM control. 

Maybe I'll make it a contested wisdom roll, where the beast and the PC are fighting for control. First roll is the DM rolling vs passive wisdom to start the transformation, second roll is contested as the PC has worked out what's going on now (and they can opt to lose, if they want to).

At any rate - I'd not let any player control a werething, because as you said, it would be unbalanced. The "Free Feat" would be surviving death, at the cost of the world around them. I'd make sure those costs start to add up!