This might be very unpopular, but there are several reasons why I think Tonelico should be renamed as either Toneriko or Aesc.
First of all, トネリコ (toneriko) is a Japanese word that means "ash tree". While "tonelico" isn't a real word; not even a name in any real, actual, existing language or culture.
I suppose Tonelico became the more popular romanization due to the already popular existing game Ar tonelico... which TYPE-MOON or Fate series isn't involved with. "Tonelico" in said series means "holy tree" in the fictional Hymmnos language, BUT again, TYPE-MOON and Ar tonelico series are not together, so Hymmnos language or Ar tonelico universe are NOT canon in Fate.
The character was named after the World Tree (Yggdrasil) which is an immense, sacred ash tree. Since the setting of FGO's Lostbelt VI is in Britain that is cut off from the rest of the world (meaning that Japan probably doesn't exist there too), having a British fairy with a Japanese name (with a weird romanization on top of that) doesn't quite make sense. Which is why the official localization opts for her localized name "Aesc" (that means "ash tree").
Let's just imagine it having a fully British character saying "My mother loved the cherry blossoms tree when she gave birth to me, so she named me Sacula". Not the best analogy, but it is similarly silly.
Of course Fate already had its share of naming issues (like Altria that's supposed to be derived from Artorius instead of Artoria, and how Olga Marie was initially localized as Olgamally in the anime... or even how 妖精騎士 is translated as Tam Lin instead of Fairy Knight), but so far I don't think I've seen any of official media using the romanization "Tonelico" for her name.
I understand well how this might be unpopular, but I just find "tonelico" a bit silly considering the points explained above. If someday they'll say the official name is Tonelico (like how silly Altria is) then fine.