First: I am sorry if you found my questions insulting, this was not intended, I was merely asking out of interest. I am a bit worried because I interpret your reply a bit harsh, but maybe you are just tired of defending your ideas...
...or maybe I am a bit tired and interpret you wrong. No worries in that case.
Second, I am not that satisfied with the answers:
A) Ok, you say that AirElementals might be too powerful. I do not disagree with you on that. So they could be balanced by changing their stats. You choose a different way by leaving them as they are, but making their Fire/Water/Earth counterparts more accessible. Fine and diversifying. I was just not sure why you preferred the one way over the other...
B) I assume it is less diversifying, since it adds more symmetry, and symmetry is order, and an ordered system is less diverse in the sense that chaos is diverse. Example: Changing Abysia by turning fire resistance into cold resistance, heat aura into cold aura, fire picks into water picks, cold preferrence instead of heat preferrence would not add much diversity to the game, IMHO. Sure there would be further differences, since not every fire item or fire spell has an ice or water counterpart, which is due to the broken symmetry (i.e. diversity) of spells and items.
This is also the case with the Elementals: FireElementals are susceptible to Cold, but WaterElementals are not susceptible to Fire, hence effectively countering the one versus the other needs slightly different tactics than just replacing a water mage with fire mage. I like that, since most other games would have them symmetric in susceptibility, but not in Dom2! Thus I was just wondering whether adding a fire suceptibility was the only way of establishing the balance (since I do agree that IDs where too strong in comparance), and whether you considered other ideas which were neglected in the end...