While the SMN doesn't "need" a complete rework, I wouldn't be opposed to it.
At the bare minimum just align the mechanics on the current SMN so that it doesn't feel like a Frankenstein of stapled-on mechanics anymore, which funnily enough isn't that much work.
- Align the way Bahamut and Phoenix work. Condense both of them into a Trance and have Bahamut also replace Ruin 3 and Outburst with something fancy like Phoenix does.
- Solve the demi-puppy behavior and the egi-ghosting.
- Replace Deathflare with Enkindle Bahamut/Phoenix once unlocked.
- Increase the cooldown from Trance into 1 minute.
Voila, current iteration is fixed and behaves like the beloved HW SMN again, except DWT is now updated to have Bahamut at 70 and Phoenix at 80.
Some further improvements would include (but these would be personal requests and not really fixes)
- Decouple Wyrmwave/Scarlet Flame from the SMN GCD. Just have the demi do it automatically each GCD.
- Have Enkindle Bahamut/Phoenix work like Pilebunker from MCH. Have the Demi do it automatically when leaving the battlefield, or leave when you press it yourself. Increase potency to match this.
- Return Shadowflare