Does it belong to Gabriel Agreste? Did it belong to Mrs.Agreste before she disappeared? Well, that's kind of the thing, we don't know and as for me, I don't think it's either of them, and allow me to explain.
There isn't to much info to say it belongs to Gabriel and there is some stuff out there to proof he isn't Hawk Moth like we all assume. I also don't believe it belongs to Mrs.Agreste just because the said Miraculous was in front of her photo, that could just be the way it was placed and organized. And how did Gabriel get the Mircaculous and the book on them? Well, Master Fu did say he lost the Miraculous and has been looking for the book, that could just mean something happened in the past and it was all an accident that can happen sometimes, the best example I can use of that is Teenage Mutant Ninja Turtles: Next Mutation, where the female turtle Venus was seperated from the others at the beginning. Most likely Gabriel found both and it was all a coincidence and he was trying to figure out the codes in the book, it is said only Master Fu can read it.
Now who do I believe is really going to get it? A new character? Maybe, Season 2 is said to have some new characters and I wouldn't mind seeing that come on, that would be cool. But I also have another character in mind, and it is one we have seen already: Prince Ali! Yes, the crowned Prince of the Achu Kingdom.
Some aren't to crazy about this idea, but I have also seen some people who are interested by this idea and I may have some ideas as to how, it took a lot of research to figure things out, but before I get into that, lets look at some evidence that was shown in his debut in "Princess Fragrance":
1. Prince Ali was tweeted about sometime way before the Princess Fragrance episode. Usually characters that are revealed sometime before their debut is usually bound to appear again and have rather important roles later on.
2. When Prince Ali had Tikki, he placed her in his pocket in his jacket. Just the fact he put of all things in their, it was a Kwami, most likely a form of foreshadowing. During the scene in the car, Ali heard Tikki sneeze and he did give a suspicious look as if he was realizing something was up. What also helps with this is that the Peacock Miraculous seems to be a brooch, and normally where would you put a brooch? In the pocket Ali had Tikki in.
3. The best piece of evidence, Ali has some similarities to Adrien: Rich boys with a strict and wealthy life style, but aren't very happy about it. This is definatly enough and it really says a lot. During the whole incident with Princess Fragrance, Ali was actually shown to enjoy the danger and got some form of excitment over it. You can definatly see this when they must jump from a window and slide down Cat Noir's staff, Chloe and the Chaperone were scared out of their minds, but Ali was having fun. It would make sense to give him a Miraculous to be a hero and escape those duties like Adrien does.
Now, what other things can help prove this, like I said, some research was needed. It is said Ali's name comes straight from the story "1001 Nights" or as it is known in English, "Arabian Nights" and in ths story, there is a character that goes by the name "Ali Baba". Ali Baba comes from the folk tale " Ali Baba and the Forty Thieves" and the story is actually performed in many media, especially for children (the more violent aspects were of course suppressed), in this story Ali Baba discovers the secret of a thieves den and later he keeps the secret of the treasure. All of this helps with proving Ali is a future Miraculous Holder, besides the names, the "secret of a thieves den" could be a small reference to the Miraculouses, and Ali may keep it a secret and want to help in any way he can as he is that kind hearted of a person and Ali Baba had a slave-girl who helped him the story and he gave her his son in marriage, maybe in a way, he takes a big interest in Rose and she helps him with his troubles and eventually they become Miraculous Users (see my theory on Rose getting a Miraculous).
Another thing that helps me with this is that Ali seems to Indian or something along those lines (I'm bad at figuring out a person's race sometimes) and given where Ali comes from, it seems Peacock's are definatly a common thing in decorations in that culture which only strenghtens my theory a bit.
I know this is a big theory, but it was the best I had and I had a lot to say to help prove it as I strongly believe in Prince Alie being a Miraculous User. If I am right, Awesome, if I am wrong, oh well XD.
(And before anyone says anything, I am not on board with giving Alya a Miraculous. She is a cool character and awesome friend but she is mainly meant to be the best friend of the main character and plays the pick me up route and doesn't need to get involved in the action)