I'm a firm believer of Itto x All and Benny x All, I just love them so much and think they deserve their harems. I've always been into the rarepair scene so like ZhongItto and Chennett were my faves. Then one day I was redoing my bio and ended up with "All x👍 All x👹" next to each other and it clicked. Why tf wasn't I shipping THEM? SO YEAHHHH- It really just began like that~ Then I began my reasons for them meeting and how well Benny, someone who found themselves outcast in Mondstadt (cuz of his bad luck), could relate to Itto, someone who was also cast out from Inazuma (cuz he's an oni)
Then because I just LIVE FOR INCEST, then I was like 'ya know, their hair looks kinda similar' and then saw how Benny always has his goggles on and went 'he totally has oni horns under those goggles' and then started this whole thing where Benny finds out he has horns when he's young, goes to the library and dreads all the books on how onis are monsters and cause havoc, and then begins to blame his bad luck on the fact that he's an oni, 'it's part of his nature'. Then eventually the traveler takes Bennett to Inazuma cuz BENNY BE ON EVERYONE'S TEAM- LET'S BE REAL, and he meets another oni for the first time, Itto. Benny sees how happy Itto is, how he's surrounded himself with people who actually appreciate him, something that Benny longs for. Sooooo he joins the Arataki gang ^^
Later on, Itto eventually introduces Benny to Granny Oni and she's stunned because she remembers Benny from when he was a baby and pulls out a picture of Itto holding a tiny little baby Benny and she tells the two that she was originally going to raise them together but one day Benny disappeared, and she didn't want to make Itto sad by telling him his baby brother was gone... BUT YEAH! Long lost brother Benny~~

Anon, I need you to come forward and kiss me on my fuckin lips and get married to me. HOW IS ONE PERSON SO INCREDIBLY BASED????
BUT GOD- the whole "You won't leave me too?" thing is SO GOOD. Like yes, he would 100% have abandonment issues, and WEAPONIZING IT? GUHHHHHHHHHHHH- I'm gonna CREAM
He keeps trying to mark his partner again and again, making it so they CAN'T leave him, holding them down despite being smaller... He's young so his stamina is neverending, just round after round of crying and biting and making them promise they'll never leave. Promises that no matter WHAT HE DOES they still won't leave. Then he can get rougher, maybe even more possessive, using his pyro power to maybe even brand his hands into their hips so they'll always be reminded of him <3333

I don't actually mind mpreg! I love breeding kinks~
But their child would probably take after Itto more. Oni genes would most likely be dominant over human ones... I've actually thought about their kid a little and they would probs have the short hair, thick eyebrows combo. Super rowdy and bubbly like Itto, but also very adventurous and sweet like Benny. Bennett's bad luck would 100% get canceled out my oni genes and their kid would probs actually have AMAZING luck. I think he'd have Benny's green eyes but cute little red horns~
I think their kid would deny having a favorite, but is always very clingy to Benny and veryyyyy protective of him. We talking he even gets a little jealous when Benny gives Itto kisses and doesn't give him any. He bites Itto sometimes to show his displeasure but very much enjoys playing with him and going beetle hunting with him. (He also inherited Itto's bean allergy) There's a lot more I wanna say but I don't know what specifics to go into LOLOLOLOL