If you guys want difficulty that doesn't essentially translate into enemies close from impossible to kill that one hit kill you, then you will have to seriously consider the most inconsiderable, sinful and criminal action to do in your lovely space ninja game. Something so abject, people here are afraid to talk about it and strongly believe they will get arrested for it. It's like the Voldemort of the warframe forum ! What am I talking about ? I'm talking about nerfs. Ooooh ! Spoooky !
Most of the issues we have with the game's current balance are, in my opinion, and indirect consequence to the effective hp gap between frames. Super tanky frames like Nezha or Baruuk can reach an insane amount of effective hp thank to damage reduction powers. Some frames don't have anything DR related but can compensate very effectively through other reliable means. Either by very high base stats (Inaros), very effective regeneration (Oberon, Wisp, Wukong, and Inaros again) or special mechanics that can effectively make them invincible (Limbo, Revenant, Rhino).
These frames pretty much shaped the endgame we have today, and had an even stronger impact than our "beloved" nukers, to the point where anything you fight at sortie level can down most squishies like Banshee or Nyx in very few hits (or a single one) in the most unfair ways. Because high survivability became a must have, this pushed players more and more towards high health and armor frames, or frames who can cheese the whole thing effortlessly (Limbo is very well known for that kind of stuff). As a side effect, shields progressively became useless.
We asked for more difficult content. DE delivered through content such as Sorties and Arbitrations. But we chose to cheese that difficulty with tanks in return. As a result, the game is getting more and more balanced around frames with insane survivability, to the point where everything gets reworked to be tankier. If you don't believe me, just consider the fact that Ember got her Overheat power back in some way.
This is why it gets very urgent to balance the effective hp between warframes so we can then balance the game's content better and make actual "endgame" missions that wouldn't translate into squishies getting perpetually trashed while tanks can pretty much shrug off everything. This effectively means nerfing a lot of survivability based stuff, but also nerfing heavily the enemy damage curve, along with buffing low survivability frames.
The biggest offender when it comes about boosting effective hp is, of course, damage reduction. Consider this : 90% DR is the equivalent of increasing your maximum hp by 10 ! When I put my warding halo on my Nezha, I'm effectively increasing my health by 10 until the halo breaks (and most of us Nezha mains know how durable that halo is). Now take armor reduction into account, add an umbral build and two arcane guardians, and you will get a frame that can reach above the 40k hp treshold, which is absolutely mental yet very needed for the current difficulty curve. This is why I'm in favor of hardcapping DR to 75% instead, which represents a much more reasonable x4 effective hp increase.
Heal based mechanics and Iron Skin might also need turning after that, but that's nothing simple value tweaks cannot fix. What would be harder to balance however is the more special kind of survivability powers. Powers such as Mesmer Skin for example (I heard it can get really crazy), or Limbo's rift mechanic. These powers may need to be adressed individually.
Just nerfing survivability wouldn't fix those who struggle to stay alive to begin with. This is why I think base stats of all warframes should be looked at. In the current metagame, low armor is usually a hard flaw to get around with, so I think all armor values should be buffed to at least 150 as a minimum armor value. That represents 33.33% damage reduction on health : not much in the end to be honest but with an enemy damage nerf it can go a long way. Shields also need to remain viable, and we all know how much shields lost popularity since the introduction of sorties. I think raising the shield base stat for all warframes could do wonders, as well as releasing some specific mod to helps shield tank. Adaptation is supposed to be the one thing you pair with shields (like health pairs with armor), but it doesn't work as nicely as everyone think it does to begin with due to its buildup mechanics. We probably need something else.
Of couse these suggestions wouldn't be meaningful without a drastic nerf to enemy damage output ! A very needed thing when you consider that some level 100 units can one hit kill Banshee Prime through a full vitality and redirection (yes, it gets really crazy). It's basically all about making the hp gap between frames narrower so we can balance the enemy damage output much, much more effectively and in the end get the same horde game you guys appreciate, but without anyone getting one hit killed unfairly. After all, squishy frame players kinda want to enjoy endgame game too, and they shouldn't have to do fancy building shenanigans just to make their warframe 's survivability work. Also, since we nerf the damage output of enemies a lot, then it would just mean that 90% DR or really busted survivability isn't as much of a requirement anymore, thus justifying the hardcap of DR and several more changes.
After all of that, I think It would be much easier to make a well balanced endgame missions that allows all warframes to be useable. It's maybe still not enough tho. The warframe endgame balancing is one hell of a problem that was ignored for way too long and it all stems from survivability powers first. Anyway, changes are needed, and not just nice buffs. We can't just keep buffing stuff. Repetitive buffing is what brought us in that situation to begin with. It's time to bring out the nerf hammer... And it shouldn't be that bad if it's done well, I promise.