Looking at the bottom 20% of frames by cumulative usage, most of which I play often, a few stand out to me as potentially benefiting from a rework. Might have some bad takes, so feel free to tell me how you play them! Always happy to learn something new about frames I haven't clicked with.
Need a refresh
Atlas: very tied to weapon selection which makes SP Circuit dicy. I'd like to see more innate power on his kit, or a change to petrifaction that improves its damage buffing, maybe with armour removal.
Equinox: augment syndrome. Some of them could be passives or otherwise extended (e.g. Energy Transfer could work on Pacify & Provoke too). Major systems rework could let her mod Day and Night separately, which would be nice as they both like different stats.
Chroma: funny damage man. Got a good buff with Circuit scaling his Spectral Scream (but not Effigy! what!) into actual strong damage abilities. The "keep two buffs rolling" playstyle is bad though. Spectral Scream could be a one-handed single cast that shoots actually good projectiles. Would also be nice to have Vex Armour store its buffs for recast, for accessibility reasons. Let Elemental Ward be recastable. Otherwise give him an augment or a passive that lets Ward and Armour refresh on some condition. Why does Gyre get to have all the braindead pew pew fun?
Could benefit, arguably fine
Gara: due to how full and synergistic her kit is, hard to fit shield gating on her build without Decaying Key usage. Good at health tanking, but that has scaling limits. Mass Vitrify deserves a buff to bring it in line with how easy Snow Globe is to scale infinitely.
Oberon: his damage buffing is very conditional on enemies reaching his garden. Very harsh range (235%!) and strength requirements if you want a fullstrip build without giving yourself RSI by double-casting Hallowed Ground constantly. Reckoning being able to blind on kills within a duration would be nice. Renewal could benefit from shield generation due to how healtanking falls off.
Lavos: please give him scaling survivability without dipping into Muzzle Flash or Resonator, or Arcane Trickery.
Trinity: absolutely horrible to play with the agonisingly short buff durations; this one is definitely personal preference though. Might benefit from damage buffing such as EV applying a small damage vulnerability in its area, or Abating Link being a target-radial armour strip that can be recast.
Out of the other low-usage frames, who would you like see refreshed or reworked?
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
Good points about Oberon. I just feel kind of bad about Renewal being my helminth slot for long runs; I ditch it for Condemn. And yeah it's hard to know what to tweak. Smite is very underrated, especially on Nourish builds and in Circuit.
Yeah, Equinox feels like a prototype frame who either borrowed from her contemporaries in a weakened or more conditional form, or else gave her abilities to later frames in a buffed form. On the other hand it does mean she has an extremely versatile kit. Red Mage energy.
Since there is the Phoenix Renewal for the health side of things, maybe Renewal could get another augment that adds some kind of shield replenishment to the ability. I feel like DE would probably make it slightly too conditional to be super realistic in actual gameplay, (like every enemy killed on Hallowed Ground restores X shields while Renewal is active) but I've always thought augments should lean more into slightly changing the way an ability works rather than just providing a buff. When it provides a buff, it ends up feeling mandatory in builds, and it can get annoying that it wasn't just baked into the ability to begin with