I got this idea that I would love to see in warframe, and could easily fit the lore and gameplay mechanics, would add an element of surprise and challenge and variety
I envision a group of tenno on a (random) mission while suddenly a lotus communication comes in stating that unidentified hostiles approach at high speed. The tenno can feel a collision not far from them, and the whole place shakes, as critical alert state is engaged. As the tenno get closer to the source of all this they start coming across some ruthless and rugged warriors, veterans of many battles, not to be trifled with! These are raiders who steal from grineer and corpus alike, and sometimes even from the ancient orokin derelicts. They are hostile to all factions but their own, and seem to live in groups or packs, scouring the system for riches and booty. I envision them riding some sort of speeder vehicles looking somewhat like bikes, but not limited to this, some of this raiders would be on foot, ofc. I see a pirate-ish feel within the warframe visual language would fit them, mostly if it's leaning to death imagery, like skulls and bony pieces (but not in an exaggerated way, of course).
With each raid there would have to be a boss or mini-boss at the end, which would drop blueprints/rare resources/rare mods/loadsa credits or maybe something better... honestly I haven't thought too much about the rewards. If they were to get weapons of their own then blueprints for those would probably make the most sense... I envision their gear to have a worn out look, to reinforce the idea that they've been fighting for a long time, throughout the system.
These raiders are supposed to be challenging opponents so, they should be a bit stronger than regular units, and some of them should have special abilities, and some should even be able to do some acrobatic moves.
None of this is particularly new or original, it's really just something I think would help make the warframe universe a bit more alive...