A concept like this would not function well within the environment of FFXIV. The main issue above all else is that, given that each of these choices is ultimately providing different DPS values, players will do the number crunching necessary to figure out which route is the "right" route. It's possible that certain routes have some sort of niche value, such as the barrier-generating path during prog, but what would happen is only 1 route would be viable 95% of the time. Just look at the pre-ShB egis for example. Either Ifrit or Garuda was the pet you'd use. Using the other was just an objectively worse decision, and Titan was exclusively a soloing pet. You'd be recreating the one thing that was properly fixed with SMN. And if your goal is that all of them and the combinations ultimately yield the same exact DPS values, then that is an astronomically difficult goal to balance when you're trying to balance 6 different summon phases and 7 different combination effects. And also, what would even be the point? Would it not feel incredibly empty to have a bunch of different choices that don't ultimately do anything different?

The inclusion of Ramuh, Leviathan, and Shiva summons is something that feels rather doable with this new landscape for SMN, but however they're included in the future will undoubtedly be just as structured as every other job in the game, such as having the SMN use them instead of Ifrit, Titan, and Garuda after Phoenix's phase, and then reverts back after Bahamut's phase. It's the most common suggestion I've seen people bring up and one that fits within the mold of the game.

Having said all that, I do think there's room to create some aspects of choice in SMN's kit to help give them a lot more flexibility. The suggestions I've brought up before would be two abilities added early on in SMN's leveling tree: one that can only be used when you have no gemstones to summon with, and Carbuncle bestows upon you a random Gemstone that lets you put off your next Bahamut/Phoenix phase; and the other which consumes all remaining gemstones to have Carbuncle perform an attack with a potency that increases based on the amount of gemstones consumed (the potency would be lower than each phase is worth, but gives you a way to make use of those unused gems if there's been a pause during the fight. Technically, neither of these concepts need to come from Carbuncle, but I also would like to see this concept of Carbuncle as your utility holder further expanded upon beyond just Radiant Aegis.