Fire I, Fire II, Fire III

[Zojha]

Exactly, it's reverting to the old-school spell naming system which actually makes better sense referring to spells with individual titles[...]

The numbered system makes a whole lot of sense if the spell is simply an upgrade without much mechanical changes. Fire III innately implies it's the same as Fire I, just +2 and thus more powerful. And for thunder, that works great - Thunder I, II and III all work the same way, you just sacrifice cast time for higher potency as you go higher.
Rule of thumb: If you can remove the old spell from your hotbar when you get the new one, the numbered system is suitable.

Looking at the fire spells, tho, Fire II is quite different from Fire I. The numerical system is just confusing there, because Fire II is not just a better Fire I - and the same goes for blizzard, which I had to explain to more than one newbie in the past >_> I'd actually go for a change there. Whether to the suffix system or to something else... /huh. Not my concern, I don't care about nostalgia, I only want clarity there :P