Being realistic about current SMN, what we need is freedom with Gemshine spells and summon abilities and that those summons produce some kind of effect. I have my ideas of changes. For example:
1) Merge Fester and Painflare into a single ability with fall off damage. Move them from spending Aetherflow stacks to an ability with 2 charges, each charge taking 15 seconds to recharge.
2) Delete Energy Drain/Syphoon.
3) Delete that useless Gem gauge.
4) G, I, T still can only be summoned after Bahamut/Phoenix phases.
5) Every time you summon G, I or T you get an Aetherflow stack. Aetherflow gauge have a max of 3 stacks, which is the max number of times you can summon in each Bahamut/Phoenix cycle.
6) Add two variations of Gemshine spells to each summon. One with cast time and the other insta cast but lower potency. If no summon is out, those become Ruin III and Ruin II respectively with lower potency than their elemental variations.
7) Ruin IV becomes the upgrade of Outburst instead of Tri-disaster. The elemental AoE versions will replace this one.
8) You stay attuned to each element for a certain period of time (i.e. you can only use elemental gemshine spells for this period)
9) You can summon another primal anytime to change elements or just resummon the same one.
10) Having at least 1 stack of Aetherflow enables a new ability (Tri-disaster) that does AoE damage. The potency of the ability's damage depends on the number of Aetherflow stacks.
11) Make the summon's abilities more specialized. For example: Ifrit's better for single target; Titan's for direct AoE damage and Garuda's for ground-effect DoT damage.
12) Make it possible to choose between Bahamut or Phoenix to summon at end of the rotation.
13) Return Bahamut's Ahk Morn and Phoenix's Revelation to be 2 shots per phase instead of just one.
14) For each stack of Aetherflow available, Astral Impulse/Flare and Fountain of Fire/Brand of Purgatory will have their potencies increased by a certain amount.
15) When Bahamut/Phoenix phase ends, Aetherflow gauge resets and we can summon G, I, T again restarting the rotation.