A couple of ideas.

WEDDING SURPRISE

So, I had that idea before where a girl kills herself, probably by hanging, in front of her boyfriend and he's all sad and confused because everything was so well between them, but then his living girlfriend appears and it turns out the one hanged was her twin sister he didn't know existed, and they both laugh at such an ingenious prank and how he totally bought it. Of course no concern is given to the dead twin's life, who was only happy to take part in such a fun prank.
So now it occurred to me that this idea might be more solid if it happens on a wedding. The story would start describing a normal wedding up to the point of "Will you marry this woman?" "Yes" "Will you marry this man?" and instead of an answer the bride takes out a knife she was hiding, smiles mischievously and cuts her throat with blood pouring onto the groom's face and her white dress... hmm, maybe the groom is also wearing a white suit just so that it looks nicer when blood pours all over it?.. anyway, the groom is in shock, kneels over the dead bride and wails all confused as everything was going so well between them, so why would she do such a thing? But then the real bride comes up, maybe unnoticed by him at first comes from his back, puts a hand on his shoulder and says "so are you done weeping, can we proceed with the wedding now" and he stands and sees her and is all confused, and she's like "oh, that? That was my twin sister, lol. You've totally bought it, haven't you? Ha-ha-ha-ha!" and as he realizes what happened he laughs too, what a fun prank it was indeed. And they continue the wedding, disregarding the corpse and the blood still left of the broom's suite (actually, maybe the twins only bought one wedding dress, so the real bride takes the blood soaked one from her dead sister and then they proceed with the wedding).

ANGEL

This one is rather vague. Or rather it has a premise, but then can go a bunch of different ways, of which I'm not sure which to choose. Not all of them even involve guro.
Anyway, it all starts with the protagonist, who is into little girls (and in case it is a guro story, he's also into guro), but lives in the world not unlike ours where it is totally illegal and unacceptable, having an especially stressful day (whether he's just not in a good mood today or he got fired from his job and his girlfriend left him and whatnot) and trying to relax by sitting on a bench at a playground watching children play. Maybe somewhere on the edge of his mind a thought appears to just fuck it all and have his way with one of those little girls. But before that thought can completely form one girl comes up to him by herself and tells she is his guardian angel and it is her job to save him from committing such a sin, and her way to do so would be to let him have his way with her, the angel, instead, and thus not harm any human girls. It is not common among angels to solve that problem the way she does by letting the guarded person use them instead of human victims, but she in particular sees nothing wrong in it. Also she could probably materialize in any form, be it girl, boy, bird, chair... but she took this form of a little girl basically from the protagonist's dreams.
From here it can go a bunch of different ways:
A1) He brings the girl home, fucks her, she leaves and he never sees her again, his stress is revealed though and life changes for the better, neither the protagonist nor the reader ever learn if she actually was an angel or not.
A2) Same as A, but it does reveal that she was actually an angel.
A3) Same as A, but it does reveal that she actually wasn't an angel, but instead just an compassionate and understanding child who wanted to help the poor guy. Well, she was an angel in a metaphorical way anyway.
B) Same as A, but he also kills her. Options 1-3 apply the same way. And even in option 3 she is totally willing and happy about helping the guy to reveal his stress.
C1) It is revealed from the start that she is a real angel. And he doesn't just get to fuck her for one night, but instead gets to keep her. With an angel by his side, he finds new will to live and his life turns for the better.
C2) Same as C1, but he doesn't only get to fuck her, but also to torture and kill her as much as he wants, as being an angel she can regenerate and revive.
C3) Same as C1, but the angel girl also has a power to ... well, I guess the way to put it is to control people's perception. The way she convinces the protagonist that she is an angel is, while they're still at the playground, she reveals to him that she is actually totally naked (and maybe even has angelic wings too). And he realizes she has been naked all that time and it is not like he didn't see her being naked, but it just didn't register as something notable. So if she has not let him perceive her nudity and then left, a moment later he wouldn't be able to remember that she was naked, or more like when remembering her it wouldn't occur to him to even question if she was naked or not, because it's not something you would normally question when remembering a random encounter like that (same with if she has wings). Like, think of any memory of yours that involves other people and unless clothes by themselves play an important role it it, you probably don't remember what they were wearing, and actually you only assume they were wearing anything at all because you think that if they weren't it would be notable and thus you would have remembered. But the trick with the angel's power is that it makes it not notable. So with that power they can just go have sex right there at the playground and people around would not perceive it as anything worth paying attention to. People can even come up and start casual conversation with both the guy and the angel girl. Like other children might come up and ask the angel girl if she wants to play together with them and she's like "no don't you see I'm busy having sex" and they're like "ah, okay then" and return to their games. Then the protagonist can also bring the angel girl to his workplace to use her to reveal stress he gets from job, and no one would question bringing an outsider to work, though they might complain if being busy having sex with her prevents him from actually doing his job. It can probably go as far as, let's say she walks through the street in all her naked glory and with a dildo inside her pussy and the dildo falls out and she asks a passer-by to pick it up and they do and hand it to her like it was nothing out of the ordinary. Or maybe even as far as she can ask a stranger to fist-fuck her and they would still see nothing out of the ordinary, might still refuse because they're in a hurry though.
C4) Same as C3, but he doesn't only get to fuck her, but also to torture and kill her as much as he wants, as being an angel she can regenerate and revive.