Makes me think of a Kabbalah Golem of sorts. So lets work off that!
Passive: Scrap Body. Surplus ammo is picked up and converted into health. (Procs after any sort of ammo mutation)
Ability 1:Void Tendril. Sweep the area with long tendrils born from the back. These tendrils deal damage, increasing in damage and add elements depending on what it "picks up". (Example: Hitting Armor = puncture damage. Shielding = magnetic. etc)
Ability 2: Lock/Unlock. Puts Warframe into a "heavy armor" mode with increased shielding and armor at the cost of speed. Unlocking gives a momentary boost of Health and shield regen.
Ability 3: Fluid Form. Momentarily transforms into a liquid state, dashing about and damaging anything caught in the way. Doing so absorbs some of the stacked Items absorbed and converts it into damage resistance for those elements. If activated while in Locked form, a shockwave is released from the starting point, damage is increased, and the unlock buffs start.
Ability 4: Keter. Summons power from the deepest roots of the Void, surrounding the Warframe with mini-portals and augmenting weapons. Primary weapons are coupled with Artemis bow arrows fired in volleys. Secondary fire is met with aid from Mesa's Peacemakers. Melee attacks send out multiple small waves from Excalibur's Exalted Blade.