Oh, wow. I've been out of the loop with Warframe for quite awhile, and this list is an absolutely wild read to me. Mostly Chroma being on it.

I main Equinox, but her entire kit could really use some tweaks.

Her passive is paltry if you're actually using Night4, but if it let you pick up health orbs without being hurt like Synth Fiber, it'd be pretty great.

1 decays instead of persisting across the direction which just kind of sucks. Duality can use archgun, which is a nice contrast to Wuclone, but is bugged to consume all reserves of archgun ammo and wastes time reloading.

If Day2 affected bosses and punched through overguard, it'd be perfect. Augment just doesn't reach far enough.

If Night3 wasn't such an energy sink, I'd use it. No comment on augment.

And 4 preventing energy regen hurts, because you always want it on. The augment ought to be baselined.

And if we were getting spicy, I'd go so far as to suggest that 3 increases Orb Drobs depending on which form - Day granting Abilities from affected teammates a chance to drop a red on kill, and blue from enemies affected by the night aura.

I totally get what you mean with health tanking falling off, but if Renewal renewed shields it would also kind of clash with Oberon's bread and butter Hunter Adrenaline builds. He's one of my favourite and most played, but I know hit kit is very clunky, and if you want to do anything active (lots of 2 and 4 casts for frequent armour stripping, or big range Hallowed Reckoning builds) he gets very energy hungry very fast, and you can't always guarantee aggro from enemies to get energy back, etc. He's needs a modernising touch to smooth out his gameplay, but I always get very torn on what should be changed. Renewal having a drain cap like Gloom seems like a no brainer, Smite is actually a cool ability with some fun niche synergies so I wouldn't want it changed drastically, and the combo of Hallowed Ground and Reckoning is fine, just clunky and expensive. He suffers a lot from wanting all stats, and sometimes struggling to weave in desired augments. He already feels great to play in the sense of bringing so much utility to a team - Phoenix Renewal, extra armour, status immunity/cleanse, rad procs for soft CC, armour strip, damage buff with Smite Infusion - so I'd only want tweaks to be small

Trinity desperately needs a modernising touch too. There isn't really any justification for her short timers anymore. I really enjoy playing a min duration Marked for Death Energy Vampire nuke build, but I know it's super memey hahahaha. If I ever get her in Circuit paired with a weapon that can prime with Viral well, I grab her and take that build every time. Her Well of Life is nice in theory but another jank ability that gets Helminthed over every time, so it could probably be reworked into something entirely new that could freshen up her kit a lot. Her augments need another look over too - Abating Link should just be baked into Link in some way imo, and Champion's Blessing relies on actually healing allies which is impossible to guarantee, as even if you're playing support heavy and watching teammates' healthbars, you're probably using it to reset their shield gates rather than actually healing them. Vampire Leech is great at least

I really, really want them to revisit Equinox and modernise her too, because the kit is way too cool for how janky and unrewarding it feels to play in its current state. I totally agree with your suggestion about being able to mod Day and Night separately, right now it feels like making too many sacrifices to use both, and the fast pace of the game means when I want to switch between them the moment has kinda passed by the time I pull off what I was trying to do. So I just tend to run with either Day or Night depending on what I feel like and what I want to do, but other frames can do what Day and Night do better. She was my first Prime so I have a soft spot for her, and I want her to be better. One thing I will at least always adore her Day form for is Zealoid Prelate

For Oberon, I wish they would un-nerf Radiation status vs Overguard. It can provide a lot of CC and if the target was buffed by an Ancient Healer pre-Overguard update, it would remove the damage reduction granted by the Ancient Healer, now it feels about as useless as Impact for a status effect. At higher levels, the increase in Eximus Units/Ancient Healers (depending on enemy faction for current mission) tends to limit the number of enemies actually effected by Radiation, and the friendly-fire damage always felt pretty negligible, even with full armor strip, and 10 stacks of Radiation + Viral.

Kinda wish they'd do something with hydroid tbh. Love the look but the kit bores the hell out of me.