It has been a long time requested, but we still do not have Ramuh or Shiva summons. Heck, Blue Mage which is a limited, and not even a real job, got skills themed after them long before summoner. It is time that Summoner got these skills, so here is my idea.
When you go into phoenix mode, it should flip the summons for Ifrit/Garuda/Titan into new ones! Theme them after the opposite elements of the magia board.
Ice is the opposite of fire, and Ifrit pairs beautifully with Shiva. Having Ifrit turn into Shiva after firebird trance expires.
Lightning is the opposite of earth, so lets make Titan turn into Ramuh.
The opposite of Wind is Fire or Ice, so this part could get a bit confusing, and if you add another fire summon, that is THREE fire summons that summoner can do.. which I understand might be a bit annoying, but I will say this. Belias. Belias makes sense if we are going for an opposite theme. Belias also already has a model in the game, so it would work quite well. It is an esper! :3