I don't think Ahri is exactly the problem.

A lot of people have been concerned with Ahri, but since she hasn't been tweaked in a long time, I don't think she's the problem directly. I think the problem was riot screwed up the MYMU and what happened afterwards kind of left Ahri in a good spot. Now look at the current meta in the midlane and jungle and you see that it favors her in some ways.

1. Malz is no longer found mid as much anymore. He sees more support time now, which might prevent her from roaming bot, but doesn't help mid.
2. Swain isn't a counter like he used to be thanks to the MYMU. Looks like more of a skill matchup now. Or rather, it seems like Ahri can probably gank bot in a timely manner can that's helping to win shorter games.
3. ~~People forgot about Annie. Midlaners don't know how to play her anymore and I think riot's balanced her too much for bot. ~~ Edit: okay after doing a little homework, annie is going mid more often than I thought, but it seems like she lost the a lot of the burst that made her a threat to Ahri.
4. Leblanc just got nerfed and every time that happens, leblanc mains turn to Ahri. Every leblanc main is technically an Ahri main given how much riot nerfs LB.
5. Fizz has a slight advantage over her, but he's not played so much. Kind of the same with the other skill match ups. People forgot who can contend with her and give her headaches. Like Anivia.
6. I think not being able to get rylai's has hurt other midlaners ability to slow ahri down and deny her some mobility. It gave mids a bunch of stats and that slow, but it's not worth anymore.
7. Not enough point and click CC in the game anymore. That really screwed Ahri over in the past.
8. I think this jungle meta is helping Ahri. I'm looking at the meta junglers in higher elos and the amount of hard CC is sorely lacking. Ahri thrives in low CC environments.

So I'd like to propose something radical to deal with Ahri. First, give leblanc a small buff so her mains stop picking waifuichi. That'll drop Ahri's play rate a few points right there. Now for the thing riot should do but never does: help out the champs who can keep ahri in check. Encourage Annie, Malz, and maybe brand back into mid. See why swain doesn't counter her like he used to. Then maybe buff some of the CC junglers a little.

Or just revert the MYMU.

Second idea: I hate to say this, but given the circumstances I think I have to: move her movespeed boost from her Q to a second benefit for her empowered passive. She'll usually use it on her Q anyways, but it won't be up as often. As an added bonus it'll work on other spells and give her a benefit to casting an empowered spell while her HP is full. Better yet, maybe make how much MS you get scale with how many things you hit with it and what you with with it (more MS for hitting champs). That way it'll have synergy with the existing passive, you can still balance it around her Orb, and she'll get potentially bigger/longer but less frequent movespeed bursts.

Expression of skill and all that.