Fire I, Fire II, Fire III

[Enkidoh]

They decided to go with Fire I,II,III because this was the original naming of spells in FF series in the beginning. The first FF titles had this naming scheme for the spells. The -ra, -ga names came later on. They went with this naming for nostalgia reason and to make them seem more simplistic.

Exactly, it's reverting to the old-school spell naming system which actually makes better sense referring to spells with individual titles or increasing levels of potency, where as the more recent 'ra'/'ga'/'ja' system really more referred to differentiating spells between single target and multiple-target spells (the use of 'ra', 'ga' and 'ja' suffixes on spell names only really started with FFVIII - all English language FF games before then used numeric titles).

And before you pass judgement, FFXI actually used both and there were no complaints (which led to some quite convoluted spell names like Curaga VI). Really, this is not going to be changed, as SE made their choice (to be really honest, I never liked the ra/ga/ja system as it sounded too weird so I am actually happy SE went this way - but that's just personal preference.