As a summoner it is heartbreaking to see primals, fight primals, and not be able to use such primals.
As summoner is currently balanced between the three types of egi, to keep this balance it would only make sense to introduce future egis as more of a choice of vanity.
For example Titan and Shiva being interchangeable options as a Tank. Each with similar yet slightly different abilities (instead of defense boost maybe an evasion boost or even a debuff that would result in the same ability to tank)
Ifrit and leviathan could be single target (stun replaced with another stun. Think water prison.)
Garuda and Ramuh could be aoe focused (knock back replaced with bind/paralyze (with a very short duration) or instead of increasing dot time ramuh could have a small dot of his own)
continued...