My humble take on new SMN
I was a SMN main in the past, and for reference I am a Final Fantasy veteran in general, having played every games since FF1. I loved SMN in the past, as it felt like the FF11 summoner, though not as fun as it was in FF11; much less options and such. But I've always loved the more FF10 approach to summoning, where the summon isn't just a one and done type thing, and they actually fight by your side. Having said that, however, I will freely admit old SMN was jank as heck and needed a rework really badly. I will only say that I do not, at all, like the new direction they took. But I know they're not going back, so instead, I'd like to provide some criticism on the current SMN incarnation, and how I'd personally find a lot more enjoyment out of it (currently, it's my least favorite class in the game by far.)
The first thing, I think, is that as it is, there's nowhere near enough stuff to keep track of in the class. My eyes usually just glaze from how overly simplistic the class is. And the main thing, I think, is that for the majority of the game, you're just pressing one button at any one time. Until level 86, you just press Ruin/Gemshine or their AoE equivalents, with a few oGCDs once in a while. I feel like I'm spamming that Ruin button even more than I used to, which is saying a lot. To that end, I think the trait that allows the use of the other spells like Slipstream or the dash with Ifrit, should be given at level 60, not 86. SMN should not start feeling like a level 60 class when it hits 86, that's absurd. From that point, you should only be getting more on top of what's already there, like other summons (Ramuh, Shiva, Leviathan, what have you) that work differently from the main trio.
Though it's not necessary, I think bringing back at least one DoT would also do a lot for keeping you actually thinking about what you're doing. I mean, come on. The most iconic Summoner in Final Fantasy, can and absolutely does cast Bio a lot. It's one of her best spells until you get the late game summons! But regardless, one thing I would like to see, is to separate Bahamut and Phoenix. Have them share the same cooldown, but allow us to choose which we want to summon. Being given the choice between a DPS centric summon and Support centric summon would already be a more meaningful choice than "which order do I press my shiny buttons". If your party is struggling, or you're progging, you can summon Phoenix more to help the healers out. Otherwise, you can summon only Bahamut for maximum damage output.
Anyway, that's just my take, as someone who is definitely not happy with the current SMN, on how it could be improved to be more complex and engaging while still being approachable. I know a lot of people love SMN as it is now, and I respect that. I just hope to see the class be tweaked moving forward to have the best of both worlds, to give complexity without it being overwhelming like it was in the past. Take it as you will!