So many model to use, like Shiva, Ravana, Ramuh, Susano, etc. why we can't use?

I understand that exist balancement problem, so, many egi = many balancement necessary to make. So, my idea is use same ability for some egi.

example:
Ravana using same spell than ifrit (changing just spell animation, but same effect/number). a variation only in the model.

I also never understood why egi has monochrome color.

I hope that in the next expansion comes something interesting to summoner