Core gameplay problems imo: animations (make movement and gameplay lackluster to me), enemy AI (AI is dumb, moves like its dumb, shoots like its dumb, and for higher levels, are bullet sponges and still dumb), and melee gameplay.
What would make the game better for me:
1) Improved movement animations - basically, more natural movement animations for Warframes, Operators, and AI. Makes the movement feel better and makes gameplay, particularly melee gameplay, flow better. As well, more of a focus on agility rather than simply speed would be great - more lateral movement (i.e. dashing instead of rolling when using melee weapons, and being able to roll and dash to the sides and backwards even when sprinting). Different enemy types moving at different speeds and holding weapons differently would also help make the game a little bit more immersive imo, mainly any heavy enemy moving slower than light enemies, gunners holding guns at their hip instead of at their shoulder, having more enemies using secondaries, having more diversity in the weapons enemies use (there are a lot of Grineer and Corpus weapons that enemies don't use - randomize their weapons).
2) Improved AI - AI needs to be aggressive, as well as being agile. They shouldn't just aimlessly run around, but should aggressively move from cover to cover, and should throw more grenades. It would also be cool if different enemy types had different abilities or tools they could use, like snipers having a power-up shot or butchers having a heavy attack or troopers having flash grenades. In Invasion missions and other missions where you're assisted by AI, they shouldn't follow you but should freely seek out enemies. If you've ever played Battlefront 2's Instant Action or COOP modes, you'll know what I mean. I want that kind of aggressive gameplay from AI. I also want some more minibosses to show up during regular gameplay
3) I find melee gameplay unsatisfying because melee gameplay is just hack and slash. I kinda like Battlefront 2's melee gameplay where you can block at the cost of speed (you walk while blocking), yet enemies can break your block and you can break there's. And this applies to enemy AI behavior. I want melee vs melee gameplay to feel almost like a duel. Instead of merely being designed around using guns or other abilities to break prosecutor blocks, I'd like heavy attacks to do that. As well, it'd be cool to have some melee minibosses where we could have that kind of gameplay.
Those are really my main core gameplay problems. I think that if you improve the AI by changing their behavior, you can create difficulty without having bullet sponges.
On the flip side, when it comes to how much damage we take, it's hard to really balance that with the limitations of current gameplay. If enemies can really hurt you at low to medium levels, then they feel too powerful. If they barely scratch you, then they're too weak. And you have to balance how much damage they do depending on how many enemies there are. It's difficult to really balance that. I say that because the game is designed around us moving fast and steamrolling through enemies. And with enemies having a lot of AOE, it's hard to just pause behind an object because it doesn't do much to help you. If you're out in the open, then you've gotta rest on how strong your Warframe is, what they're abilities are and if they're powerful/good CC. I think the solution to that isn't to just have bullet sponge enemies who deal high damage at higher levels. It's to have hard-hitting weapons, both our weapons and enemy weapons, without having high-health enemies. And if enemies have high health, then their damage output should be balanced accordingly, especially if these enemies aren'tdesigned around four players taking them down (and they really aren't designed that way, except for bosses). I think a lot focus is given towards only balancing enemies based on OP warframes. They should also be balanced based on their health and damage. That, in combination with improvements to the AI, would help make the gameplay better, imo.
The key to improving enemies and upping difficulty isn't in boosting their health or even necessarily their damage. It's in improving their accuracy and doing basic AI upgrades like having them move faster, more aggressively, utilize rolling, and using more tools, like different types of grenades, different types of attacks, different buffs to enemy allies, more minibosses, etc. I'd say it's the same thing with our gameplay. Give the player more tools to use, and you can make the gameplay more interesting.