So according to
this Disney insider from Instagram, Disney has apparently already cast Ariel! Announcement is bound to happen within the next two months.
He wouldn't spill the name of the actress but gave away several hints. It's a singer/actress who has played in a couple of films so far, two in particular are what made Disney realize she's the perfect pick to play the little mermaid. Her casting will make some fans angry and others will be very happy about it.
Shooting is said to start in early 2019, with the movie being released in late 2020. After casting Ariel, Disney will start looking for someone to play prince Eric.
---
Analyzing those hints lead me to believe that
Zendaya might have been cast for the role! And I ain't even mad

I did always imagine someone very pale to play Ariel in live action, since that goes perfectly with red hair. And there's no sunlight under the sea anyway, so it would've made sense. But seeing how Momoa looks so badass as Aquaman, I can definitely imagine a PoC for Ariel too. And Zendaya is beautiful. I definietely picture her alongside Matthew Daddario as mentioned above.
It would be Zendaya's second role as an iconic redhead, following her portrayal of Michele "MJ" Johnson in "Spider-Man: Homecoming" - but here's hoping Disney will stick closer to the original film then Marvel did with the comics in "MJ's" case, and give her actual red hair. It's a must for me.