[{quoted}](name=Dr Mercy,realm=NA,application-id=6kFXY1kR,discussion-id=6ILmlHHp,comment-id=000300000000,timestamp=2017-08-10T03:09:43.139+0000)
That said, they wanted Older looking people for SG 2, so MF kinda fits that bill of mature.
I'll stop there for now, but I am more than willing to continue explaining things for this. There is a lot of variables that include stuff, theme, lore, background and base visuals, that need considering, and Ahri matches that extremely well. I dare you to try and suggest some other Champions that would be a better fit following the conditions I'll set below.
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They need to be more mature feeling, Lux is still a newbie, they need to feel like they're more experienced than her
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They need to be in their base skin, much more lighthearted than other champions. Not so serious.
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It needs to make sense that they are a good guy
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It needs to resonate with the champion's main audiance
I will argue this all night if need be.
Looks left, then right

1.)
Fiora: More mature than Lux. She'd have to be to earn the title of 'Grand Duelist' for her base skin/lore.
Sona: Not to particularly draw attention to... certain aspects of her design, but yes, more mature.
Leona: Oh so very much.
Taric: Again, yes. He oozes maturity and experience.
Vi: Maybe not so much on the maturity side, or even seriousness, but I'd argue for more experienced at least.
2.)
Fiora: You got me there. Fiora is serious, but probably not in the top 10% of most serious champions out of the whole catalog. There is a toying to some of her lines.
Sona: Uhh, yep. Suuuuper too many musical puns to not call her lighthearted. Beyond that, she finds a joy in the tempo of combat, not in the spilling of blood or righting a wrong.
Leona: I'm gotten again. My best gal Leona is quite the serious lady. Then again, so is Syndra. Her ambition is a fury bordering onto wrath. She's the walking (or is it floating) definition of hell hath no fury like a woman-.
Taric: Taric, even now, has a sense of levity to his dialogue and lines.
Vi: Vi? Stands for vitality. She's got a smile on her face, a laugh in her throat, and a spring in her step. She does what she does half because it needs to be done, and half because she'll have fun doing it.
3.) I actually question this one outright, because again.... Syndra, and before/after her is Jinx. It's NOT obvious that they're good, in fact it's super unobvious. Syndra works as a veteran fallen hero, but Jinx is as uncaring hero at her best points. They stretched the form for Jinx well beyond what they would have to for any of the five I selected.
4.) I question this as well, because it's different strokes for different folks. You assume a champion's visual design and how it is reached is the only thing that reflects back on the people who love/main said champion. I love Steven Universe, I adore Friendship Is Magic, and I'm currently on my sixth or seventh watch-through of Ouran High School Host Club.
I'm an engaging Tank main. Gimme a Sion, a Galio, a Rammus. Put me on Singed or Maokai or Thresh. If you want a sure win, let me walk onto the field as Leona. I want to be the first one in the fray, denying every little thing my enemies want to do like I'm smacking down a basketball saying "No-no-no~".
You (as in anyone, not you specifically) can rationalize any champion fitting into almost any number of skin themes. The biggest factor though, when it comes to who gets what (or any) skins is -timing-. Since release, Ahri's averaged better than one skin a year. Partially, that's because she was made back when champions got 2 skins when launched. Even if you don't count Dynasty and Midnight though, that statement still stands. She came out in ultra late '11. We're halfway through '17 and she's on the cusp of her sixth post release skin. Eight months between Challenger and Academy, just under a year between Academy and Arcade, and it's looking like just under a year again for Academy to Star Guardian.
There -were- and still are other champions that could fit the role or theme, like Sivir which you mentioned and I totally agree with. Skins are decided well ahead of time and balanced on what themes could be explored, but more so than that, who doesn't already have a skin in the works or just released one.
Was it really the best use of resources and time to make another Ahri skin for a group theme, and make it a Legendary at that?
Obviously yes. Ahri skins sell better than hotcakes, and I've been led to believe hotcakes sell particularly well. But this is the internet, the nexus point of where people go to complain. Allow us to have this, then. Let us argue for the 'what could of been', because it might influence the 'what will be' of the future.
When you look over the grand scheme of things, they didn't -need- to pick Ahri specifically for this skin slot, and that's what we're complaining about. Someone else could of used the space better. So now we've got a Legendary Ahri skin that doesn't do much for the base character's design space for the price slot, and we're not gonna get another Ahri skin for a long while, closing off better utilizations of unexplored options (Bestial/Ravenous Ahri, Demonic Ahri, Program Ahri, Kigurumi Ahri etc).
Oh, and Lucian so totally fits to Star Guardian. SG is based pretty heavy on the theming and designs of Madoka Magica, and do you know who's pretty dang diddly serious in that?
Homura
https://s-media-cache-ak0.pinimg.com/originals/64/44/fd/6444fd096c375fb7a06dfe96e2d5823d.jpg
Her power is even 'all of the guns' (among other certain things). Smash that together with a Tuxedo Mask and you've got Star Guardian Lucian. >:3
Edit: I will say one joyful thing about SG Ahri. I absolutely adore her new laugh. It's spot frickin' on.
https://www.youtube.com/watch?v=0AuFgZT4O-I