I don't think this would work for several reasons:
A) You are not giving Riot any kind of example of what it should look like... Usually these don't work without some kind of template or general idea of what it should look like. That's not to say 'No Chance in Hell' but generally these ideas are a lot better for some reason if Riot has a template first.
B) Dubstep Sona, I imagine, would be like a darker version of Arcade Sona, right down to digital effects. There are online websites where people will create custom skins, I'm sure someone has already done with with the Arcade Sona to the point this probably exists in one dimension or another.
C) Sona isn't as picked a support as she used to be... She was very popular in Season 2 and 3, because her heal did a lot without any kind of Ability Point item, and only if you really failed as her could you NOT scale into late game. Now, with the onset of Season 3 and non-traditional support 'supports' emerging, and with the 'nerf' to her heal, Sona isn't as viable as she used to be in the early game. More people I've seen lately are leaning towards Leona, Annie, or even Soraka. Riot won't spend a lot of man power developing skins for champions that 1) have a TON of skins already, and 2) aren't picked a lot anyway. When Riot works on skins, they usually pick 'popular' (at the time) Champions in which the skin will be popular and they are more likely to get a return in their investment.
D) In a way, Pentakill Sona doesn't seem right, but when you consider her lore, it's completely viable. She is a member of the 'Pentakill' band, which has a brutal appearance similar to that of a heavy metal band (Which I think they were aiming for) so it would only make sense that her skin for it also has a 'gothic' or heavy metal theme to it.
Overall, I'm going to not say 'Fuck you, Dubstep is stupid' because everyone has their own personal tastes, and you know what? That's perfectly acceptable, especially in today's society (as much as there ARE times I'd LOVE to disagree, doing so would be wrong.) And I'm not saying your idea is stupid, but perhaps you should take a step back, examine your idea, and think less along the lines of 'This is a great idea and almost everyone is going to love it!' and more along the lines of 'Okay, this is a good idea, how can I make it seem even better?'
As you can see from the title, I am wondering if RIOT will ever make a new skin for Sona. Some skins for Sona are just ugly and makes you wonder like," What the fuck is that!??!". Don't get me wrong that I am saying that all of her skins are ugly, I am just saying one of them, to be more specific, the penta kill Sona skin. That skin is a what the hell thing.To be honest and frank, it doesn't match her personality if you think about it. You are probably stop reading after I said that the Penta kill Sona was ugly and wondering when are you going to get to the point and tell us your idea of a new skin for Sona. Here it is right now, I am thinking of dubstep Sona, right you probably go and say "GO FUCK YOURSELF!!" or "Dubstep is for pussies" etc. Think about this, if you made something that is ugly as penta kill Sona, then you might as will make a dubstep Sona. that was my personal opinion on penta kill Sona and I am not saying that everyone hates that skin, I am saying that in my opinion that they could do better and make another skin. People might like Penta kill Sona or others might hate it.