So I had an idea about a year ago for how the show could end. It's kind of cliche, but I think it would be an awesome conclusion to Rooster Teeth's longest running series.
The Reds and Blues are finally able to really relax, now that they have no more adventures to embark on or sinister conspiracies to uncover. It all goes to hell when a mysterious soldier turns up and tries to attacks them. The guys try to fight back, but are ultimately forced to flee when they realize two things: this soldier (let's call him Reaper) is terrifyingly powerful in terms of physical strength and the equipment he wields, and he means to kill them. For reasons known only to Reaper (at least for a solid while), he wants each and every man and woman of the Reds and Blues dead. All they know about Reaper's motives is that he's seeking revenge for something they did a long time ago. He relentlessly pursues the gang everywhere they go, and even comes close to killing certain individuals in multiple encounters. Eventually someone (let's say Wash) runs off to do some digging on their mysterious assailant while the others do their best to not die. The crew is tired and has suffered multiple near-fatal injuries, but at last learn the truth upon Washington's return. Turns out the most silent of the Freelancers--agent Maine--had an older brother that no one knew about, and guess who it is. When he learned what happened to his younger brother, Reaper set out to hunt down and kill everyone responsible for Maine's death. Finally there's one big winner-take-all fight. At some point Reaper manages to cripple and stands poised to kill someone (Carolina, maybe). Sarge makes the decision to save Carolina by confronting Reaper one-on-one, blasting the bastard all the way with his shotgun. After a brief scrap Reaper disarms and impales Sarge through the abdomen with one punch, but before he can move on Sarge distracts him with a witty question (something about spiders) just as a plasma grenade flies in and adheres itself to the psycho's chest. At the same time Tucker rushes in with his sword, cuts off Reaper's arm, and throws himself and Sarge to safety just before the grenade explodes. Reaper still stands for a moment, mutters something about his brother, and finally falls dead. Everyone crowds around Sarge, but he too is deceased. A funeral is held, and as each member of the Reds and Blues takes a turn to leave the audience sees snippets of what they go on to do in the future. Soon Griff is the last one left. He removes his helmet and places it on Sarge's grave, and then departs without a word, and no idea what the future holds for Dexter Griff.