Luna's did I kill your brother joke is great, but I think there is a good chance that we will find out that she really did or will kill a brother of ours. It would be hilarious Luna laughing her ass off because she actually killed our brother.
As far as Dominico, the MC's confidence early was odd that's why I read a lot into it. Given what we see and find out later about the MC liking to be underestimated and the fact that he beat a Quasi-Capo. I just can't see Dominico as a rival, and no I don't believe Hopes ever said that he would be a rival it was just speculation based on their first scene.
lol I could be wrong, but I'm leaning more towards probably not.
If Luna for some reason does killed or will kill MC's brother, I don't know. I just don't think she'll find the humor in it like in her original encounter with the MC.
Back when she first met the MC, she could basically say that line and laugh it off because she didn't know MC at all. But now, she's basically setting her foot in the "love" territory in terms of her feelings regarding the MC.
Just feels like a completely different situation.
This is just my thought, but with the Luna we've gotten to know by the end of v0.07.2, I feel if she has done something which seriously affects MC (regardless of whether MC is aware of it or not), I get the feeling she will be more concerned, and a bit wary about how MC will react, instead of laughing it off as a joke.
Regarding Dominico, well no one knows for sure whether he will actually be the rival or not (only Hopes does).
But whether Dominico actually becomes MC's rival or not, one fact remains true; MC will have a rival, which the 4 Serpents will take both (MC & the rival) to the mountains to train somewhere down the storyline (this was confirmed by Hopes in an answer to one of my old questions "Will we ever see the 4 Serpents together in one place?").
And it really doesn't make sense for Dominico to appear so early in the storyline (as if the narrative is hyping him up a bit) if he doesn't come into the story as a major factor in the future.
While I could be wrong, only way I'm seeing the reason for his early appearance, is for him to be the rival to the MC (cause otherwise, Hopes will need to set up another character to be a rival for the 4 Serpents mountain training event. Dominico already has some sort of animosity towards the MC. Sounds like a good fit for a rival to me).
To add, Dominico looks to be pretty similar age to MC, as well as same rank of Soldato (well not in the beginning of game, but as of v0.07.2 I mean, unless Dominico ranked up to Elite Soldato between that time), factors that I think can attribute to having good rivalry relationship: 2 individuals in same age group moving up in ranks trying to outdo each other.
But as I said, only time will tell how it all plays out.