In the past, I submitted this thread as a set of concepts for frames I would like to see brought into Warframe:
Looking back, the one that I still most want to see is the "sniper frame" "Carl / Carla," however I realize that for the game in its current incarnation, this concept may be a bit bland. Therefore, as my submission for this call for themes, I offer a modified concept.
Carla and Christa, the Slime-based Sniper Sisters Warframe
The frame will consist of Carla, the player controlled sniper, and her spotter and sister Christa, an ecto-plasmic specter who is always both silent and invisible to the enemy.
I imagine this frame as the inverse of Equinox, two frames with one body, whereas this will be one frame with two bodies. The frame would treat Christa similarly to Wukong's Celestial Twin or Khora's Venari, but with more versatility owing to it being an additional human attached to the frame rather than a pet or a clone.
In terms of background, the Sisters could either be mercenaries in the same vein as Ordan Karras, or Dax soldiers originating from a swampy environment. They therefore operate as the Warframe variant of a traditional sniper team, having first been designed as two separate frames by Archimedians, but condensed into one after the Tenno and Warframe projects were merged, and finding that Christa performed much better in support of Tenno than controlled by them. As her sister's (and the Tenno's) spotter, Christa provides Carla and allies with increased critical chance, recon capability on the mini-map, sentry-removal, covering fire with an independent loadout, and enhanced stealth capabilities. However, only being semi-corporeal, Carla must use energy to keep Christa summoned and projected out from Carla's warframe.
Carla herself uses slime-like nanite swarms to manipulate her environment. Instead of a conventional ghillie suit or stealth like Loki or Ivara, she maintains a morphing globular stealth suit that not only blends her seamlessly to her environment, but even blends her environment to her. Further, by "sliming" allies and enemies, she's able to provide various buffs and debuffs to aid in her mission. For example, by causing a radial sliming, she can open enemies up to finishers, blind them, and even reset their alert level, or provide allies with a stealth boost, a decrease to enemy accuracy, and materialize ammo for allies from the nanite slime.
As a team, Carla and Christa are able to assassinate any target from any range, both directly affecting the enemy and assisting their allies with their nanite slime.
If a signature weapon or weapons were to be used for this frame, I would recommend:
A single dagger, reflecting how a sniper's knife is often as vital as his rifle.
A silenced semi-auto pistol of intermediate capability between the Lato and the Lex, presenting something of a "Mk. 23" .45cal pistol of Warframe.
A single shot bolt action sniper rifle similar to the Vectis, but with increasing crit chance as long as you hold your sights on the target.
If a novel piece of equipment were to be implemented:
A Tenno-equipable Spotting Scope that provided Crit Chance and Damage boosts to allies
A Tenno-deployable recon and surveillance drone, both for use in scouting and in buffing allies and one's self
If a new mission type were to be implemented along with this frame:
Sniper Assassination missions, which would give the player an open-world target, and provide the enemy with the tools to force players to have to engage from long range, or attempt a "sniper crawl" to approach the target with stealth.
Intelligence, Surveillance, and Reconnaissance missions to aid Tenno and/or their allies in finding targets for later missions.