It would have been too human. Even if I like Gentiana more than Shiva visually, I don't think she'd have fit as a summon. Though maybe they could have retained more elements of her design for Shiva, like a similar fringe and maybe even white robes approximating the silhouette of her outfit. The current Shiva design could even be retained for the additional ones that appear during the summon sequence; clearly distinguishing which is the lead could give the scene more focus.
In that scenario, I like the in-universe justification of the other "Shivas" being various women Gentiana granted a form of immortality to throughout the ages after their untimely deaths. Maybe Luna could be part of her harem when she shows up against Ifrit...