We have this conversation often around here. tl;dr, the devs decided to use roman numerals in the English localization and that's just what we have to live with. It won't be changed.
From a UI/UX standpoint, a suffix system would work if it were like the one used in FFXI, where the suffixes were meaningful--
- Base spells: single target
- -ra spells: AoE centered on caster
- -ga spells: AoE centered on target
- -ja spells: AoE centered on target and reduces target's resistance to subsequent uses of the spell
--so they weren't just power indicators. In that system, they
also used roman numerals to denote different power levels of spells with the same target scheme. If BLM and WHM spells used that same system, it would look something like this:
Current system >> Suffix system
- Fire » Fire
- Fire II » Firaga
- Fire III » Fire II
- Fire IV » Fire III
- Blizzard » Blizzard
- Blizzard II » Blizzara
- Blizzard III » Blizzard II
- Blizzard IV » Blizzard III
- Thunder » Thunder
- Thunder II » Thundaga
- Thunder III » Thunder II
- Thunder IV » Thundaga II
- Cure » Cure
- Cure II » Cure II
- Cure III » Curaga
- Medica » Cura
- Medica II » Cura II
However, still from a UI/UX perspective, if you wanted to just rename Fire/II/III/IV to Fire/Fira/Firaga/Firaja, then it would be
no different from naming them Fire/II/III/IV
except that then people would have to learn to associate --/-ra/-ga/-ja with the meanings of 1/2/3/4 or A/B/C/D, which would be an additional cognitive load on the user. Therefore, if we're not going to use meaning-bearing suffixes like XI uses, the more user friendly choice is just to number the spells.