I think that knowing they're being cancelled and that they're running out of time in general, not just on Sci-Fi - they should go out with one helluva ****ing bang.
Have the Ori fleet split into four squadrons with a few dozen ships in each. One goes to Dakara, one goes after the Nox, one goes after the Asgard and the largest of the four goes after Earth.
Then have everyone getting their ****ing asses kicked six way from Sunday. The fleets in orbit of the 4 homeworlds get whupped to **** and cast aside (though dealing some minimal, suicidal damage), and the Ori start landing millions of troops on every world - with yet more pouring in through the Stargate on Dakara (as the other worlds would have gate-defences).
Everything goes to **** and the only four real powers left in the galaxy get ass-raped by the forces of the Ori. They take the Temple on Dakara, Adria meets with the High Council and butchers them all. They blow the Nox city-ship outta the sky and an army of Priors sweep through their forests picking off stragglers. Tens of thousands of Ori fighters swarm through the Asgard cities as the little grey skeletons charge into the streets to meet the enemy and rooftop defence systems struggle to keep the fighters from supporting the ground troops.
On Dakara, Teal'c and Bra'tac lead the last few Jaffa inside the Temple against the Ori soliders who've taken it. They slaughter hundreds of them. Bra'tac and Teal'c are captured. Adria tortures Bra'tac to death then turns on Teal'c - trying to get his Iris code so she can flood the SGC with Ori troops. He endures it for a while but starts to feel her breaking into his mind, so he breaks free of his bonds, grabs an Ori staff and shoots himself in the head, destroying his brain, the Iris code contained therein and any hope Adria has of ressurecting him to start over again.
On Earth half the population is dead, the other half are either bowing before the Priors or are in the process of becoming dead. The major cities are all in flames, President Hayes is dead and the various NID, Russian and corporate people we're seen over the years are hunted down and butchered.
Hammond takes command of the Odyssey and kamikazes a cluster of Ori ships, detonating the Naquadriah hyper-space core and vapourising them.
O'Neill orchestrates the aerial defence of Earth with thousands of US, Russian and Chinese 302's engaging the Ori fighters in a massive battle as their warships move to sweep the Antarctic site and Cheyenne mountain. The Ori swarm his command center and he manages to beam out just before they open up on the place with the big guns.
Then SGC forces battle with and gradually get picked off by Ori troops as they move to secure the Stargate in hopes of flooding the base with reinforcements from their home galaxy. Mitchell gets seperated from the group and ends up leading Siler and a load of techies outta the base to safety. Carter gets hit bad, but not mortally wounded. Mitchell and co pop out of one of the service hatches in the woods outside the SGC but run straight into an Ori patrol and are cut down, with only Siler escaping off into the wilderness.
Then, at the last minute, as only Carter, Jackson, Vala, O'Neill, Walter and a few troops are left trying to defend the Gate and Control Room, Jackson gets hit and goes down.
He gets all pissy and starts ranting about the Ancients, assumes they're watching and starts screaming at them. Then all of a sudden he's in the restaraunt at the end of the universe and Oma's there. She tells him she can't help, but she's let up on Anubis (who's sitting in a booth) just long enough to offer Jackson a way out of the battle - to help him reascend. He *****es, argues and eventually just ignores her. He walks over to Anubis and sits in his booth and starts talking to him.
After a big speech similar to the one Teal'c gave Gerak - about the measure of a God being not in the scope of his power but the manner in which he wields it - he convinces Anubis that having one last stab at true Godhood by attacking the Ori and being destroyed is better than returning to an eternity in conflict with Oma. That he might just earn himself the right to wield his powers and at least a little redemption if he joins the battle.
Anubis figures '**** it' and starts to stand up. But Oma puts her hand on his shoulder and says she won't let him. Then she offers Jackson one last chance to ascend. He accepts, then him and Anubis turn on Oma.
The Ori break through into the Gate and Control rooms and Adria swans in to gloat and take Vala. As she's prancing around talking about the awesome power of the Ori, Jackson stands up. She waves a hand to strike him down, but all that happens is the computers explode behind him.
Outside and across the galaxy, the skies of all the worlds conquered by or under attack from the Ori blaze with fire as the battle between the Ancients and the Ori is finally begun by Anubis firing a shot into the Ori's bow - so to speak - and vapourising one of their planets before any of the Ancients even realise what the **** is going on (having assumed he was still in battle with Oma).
Carnage ensues as massive, etherial weaponry is employed by both sides. The fire begins to lose out to the white light, and suddenly - across a thousand worlds - all the various Ancients we've seen over the years pop into sight - Skaara, Shifu, Orlin, Morgan. They show up all glowy in the midst of ground-battles and vapourise the Priors and as many of the Ori soldiers as needed to get them to cower down and surrender. In space the Ori ships are torn to pieces by massive arcs of energy and vapourise as they lose core-containment.
Back at the SGC Jackson is still facing off with Adria, who's panicing as she feels the deaths of millions of her followers and the loss of thousands of Priors - not to mention the Ori themselves being driven to the brink of defeat by the Ancients.
He goes to strike her down (in-keeping with his 'she's not a child' speech and belief she's as evil as the Ori) but Vala stops him. He thinks about it for a minute and goes "A very wise little boy once told me that when the evil in a man's mind was so great that good couldn't possibly triumph, the only way to win was to deny it battle..."
He walks towards Adria - as her troops shoot through him - and presses a finger to her head. She collapses and he tells the troops the Ori have been destroyed and that they can either fight in the name of dead Gods or surrender and be sent back home. They surrender and all across the galaxy the Ori troopers who survived are cast back to their respective worlds by the Ancients.
In Celestis we see the Doci looking totally defeated, wide-eyed and shaken. Then the fires glow brighter and surge with energy as the Ancients finally reach their last little stronghold. The earth quakes and the city begins to crumble as the Ancients tear it down at it's etherial foundations and vanquish the Ori once and for all.
Vala runs over to tend to Adria, demanding to know what Jackson did to her. Adria stirs and wakes up. She recognises Vala as her mother, but can't remember anything else and has lost all but the most passive of her powers, such as healing - which she instinctively uses on Carter.
Reports start coming into the Control room of Asgard ships in orbit, which headed for Earth to assist the Tau'ri - having defeated the Ori fleet attacking their world on their own, without the assistance of the Ancients (sorta showing the Ancients are no longer needed as guardians now the Ori are gone). Walter and O'Neill start coordinating with the Asgard to restore some order to Earth. Vala sits with Adria. Jackson tells Carter about Teal'c, Bra'tac, Hammond and Mitchell, then says goodbye. He gives Jack a quick goodbye, saying very profound things about Earth finally being ready to stand on it's own and to help bring the Jaffa, Asgard, Tau'ri and Gadmeer (who apparently held out fairly well on their own during all this) together into a new Great Alliance - but that Jack won't be there to see it.
O'Neill's like "Eh?", then he grabs his chest and drops to the floor as the Stargate begins to turn.
Jackson stands over him as he splutters his last living witticism and dies with Carter holding him and crying.
Then he appears next-to Jackson as the gate finally engages.
Everyone stares and Jackson turns to O'Neill, telling him that they were the first ones to step through the gate, the first ones to open up the universe and to set all this in motion - and that now it was all finished, they should be the last to leave through it.
Carter's all "What? You're gonna take the gate?". Jackson tells her the Ancients are taking all the gates from this galaxy, having seen the horrors their creations have allowed - first through the Goa'uld, then Anubis and now the Ori. But not to worry about Atlantis because they still had 'big hopes' for their fight agains the Wraith, and because Earth has ships now and the knowledge and resources to build more - and that if they really wanted the Stargate network back, the Asgard and Tau'ri working together would 'think of something'.
They go all glowy and float to the foot of the ramp before becoming human again. They look back to see Carter, Walter, Vala and Adria watching them from the Control room. The camera shot goes to the foot of the ramp, looking into the event horizon as it glows a little brighter and intangible shapes appear and fade on the other side. O'Neill sees all 9 Chevrons lit on the gate and asks "Nine Chevron's, huh? Where we going?". The camera closes up to the both of them. Jackson looks at him thoughtfully and goes "You ever see The Wizard of Oz?" "Yellow brick road? Over the rainbow? Emerald City?". Jackson pauses, nods-n-smiles, looks into the strange new event horizon and says "Something like that" as they walk right up to it. O'Neill touches the event horizon and wiggles his fingers in it. He looks at Jackson one last time just as he's stepping through. He stands another few seconds looking into the event horizon, turns back to the Control room and Carter, looking a little apprehensive. He turns back to the gate looking even more wary of stepping through, then suddenly something clicks inside him, all his anxiety and apprehension fade from his face, he smiles and steps boldly through the event horizon.
The camera-shot changes to the whole-gate shot.
The gate disengages, the camera lingers for a few seconds and the screen fades to black.
Fin.
Wow, kinda went off on one with that. But cool, no?
