I honestly don't think we should expect to have all the element anymore, it'd give SMN way too many unique animations and VFX compared to other jobs and be unfair in that regard. It was actually more likely when the egis were their own thing and didn't give SMN specific actions for each one.

That said if they do decide to go with it, I think we can expect there to be a mechanic separating the two "rows" of summons. Like how astro now has astral and umbral pulls, I could see the new summons being unlocked during a specific phase of the rotation. Maybe have them be tied to Solar Bahamut, because obviously they're not gonna get rid of it completely ? I could see it going that way:

Instead of having solar bahamut be the first summon in the rotation for no reason, there could be a new gauge that fills up when you land hits as phoenix, that represents our umbral ether, which when filled would allow us to summon Solar bahamut as the next demi (changing his affiliation, essentially) which would grant us the new gems instead of the old ones. And after using all three you would get access to Lunar Phoenix as the demi, which would fill the gauge with astral ether, turning everything back to how it currently is.

I also assume we'd get an action that fills up the gauge with umbral ether instantly, so that we can match other jobs' burst windows during openers.

But maybe that's too similar to other existing jobs.