First, the questions:
- Is it intended to only be used by gnomes?
- How does one enter/remove the Warframe (how long does it take)?
- Can one wear armor while within the Warframe or is it considered armor? If so, what is the AC?
- If the Warframe is destroyed, how long and at what cost can a replacement be made? If it can be destroyed or damaged, how is this done and what would be the properties of the Warframe as an object?
Comments after each feature (if provided).
Augmented Strength
When you choose this archetype at 3rd level, you have finished the basic framework of your exoskeleton. This frame provides significantly increased strength and stability. When you wield a weapon with the Versatile trait in one hand, you use the same damage die as if you were wielding it in two hands. You also ignore the Heavy property of weapons entirely.
In addition, you can attempt a grapple or shove against creatures up to two sizes larger than you. Your carrying capacity doubles.
Great “kickoff” feature. My only concern would be the grapple/shove mechanic – especially if the suit is worn by a medium or large creature (e.g. a custom large PC race or a medium creature under the effect of the enlarge/reduce spell). A large creature could grapple something gargantuan.
Weaponized Exoskeleton
By the time you reach 10th level, you have completed a significant upgrade to your exoskeleton. Choose one of the options below:
Sweeping Strike: You have upgraded the arms of your suit, letting you scythe through enemies. Once per turn when you take the Attack action, you can perform a sweeping strike instead of one of your regular attacks. Choose a 180 degree arc and make an attack roll; all targets in that arc within the reach of your weapon that would be hit by that attack roll take damage as if they were struck by the weapon normally.
Death from Above: You have upgraded the legs of your suit, enabling potent leaping attacks. When you fall at least 10 feet onto another creature and make a successful weapon attack against it, the target must make a Strength saving throw or be knocked prone and be pushed 5 feet in a direction of your choosing. If the target fails the saving throw, you can use a bonus action to make one additional weapon attack against it.
If you would have taken falling damage, you take none and the target takes the whole amount as additional damage whether or not it failed the save.
I can see how this applies to falling on another creature (though it’s not clear how one would control where they land), but it’s not clear how you would actively jump up, attain height, and land on an enemy. The bonus action portion should be reworded as, on the failed save, the target is pushed 5 feet and is then likely out of reach of the bonus attack.
Defensive Augmentation
By the time you reach 15th level, you have completed a second significant upgrade. Choose one of the options below:
Turn the Blow: You have strengthened the arms of the suit to let you deflect melee attacks against you. When another creature hits you with a melee weapon attack, you can make an attack roll as a reaction. If your total result is higher than the triggering attack, you successfully deflect the attack. If you roll a critical hit, the force of the parry unbalances the opponent and the next attack against them has advantage.
Deflecting blows strains the mechanism--once you successfully deflect an attack twice in this way, you must complete maintenance during a long or short rest before you can use it again.
I’m not sure the limit/recovery mechanism is needed – you already have a cost (reaction) so it would be difficult to abuse. Alternately, if you wanted to be boring (and assuming the Warframe is your armor and/or you can’t use shield), you could make the Warframe’s arm serve the role of shield (static AC increase).
Stabilizing Frame
Beginning at 18th level, the torso portion of your exoskeleton has been augmented to provide medical treatment to keep you in the fight longer. At the beginning of each of your turns, you regain hit points equal to your Constitution modifier if you have no more than half of your hit points left and are not incapacitated.
Add the “(minimum of 1)” wording just in case…