For one thing, we would have found out that the ascended Daniel Jackson had gone back in time and caused the whole sequence of events. He would have arranged the gate addresses so that the SG1 team was guaranteed to make the right discoveries in the right order, for one thing. He would have angsted terribly over killing Sha're. He would have deliberately tipped off the Ori to the existence of humanity in the Milky Way so that a war to annihilate them would begin. He, not Merlin, would have created the genocide device. We would have had a lot fewer scenes with Daniel Jackson because that goody two shoes shit makes grown ups puke. He'd basically just be seen in the beginning and the ends of lots of episodes.
Jolinar wouldn't have been killed, but Sam/Jolinar would have been kept for the rest of the series. Sam's hand to hand combat skills would have been awesome. Jacob would have been possessed by Apophis so that she would be the step-daughter of the series' first Big Bad. She would have had a love life, that included, progressively, the greatest Tokra covert ops agent evah!, the President and a mutual affair with Orlin that was led to the defeat of the Ori. And Replicator Sam would have been a lesbian that we saw in at least six episodes. She would have been the greatest female character ever written. And not just in scifi, either.
We would have seen Jack living Joe the Barber's life, and vice versa, from the time the communicating stones were found. Jack would have personally killed the reporter who found out about the stargate way back when. Jack would have always known about NID but refused to take action. Jack would have ordered the use of the symbiote poison to annihilate whole planets of go'a'uld and Jaffa. Jack would have used alien technology to topple the Russian government. Jack would never be the butt of a joke.
T'ealc would have killed every competitor for leader of the Jaffa with his bare hands because they were insufficiently pro-human, not just Raknor and Garak, but Bra'tac too. But he would have angsted about killing Bra'tac for at least ten episodes. Ry'ac would never have reconciled and T'ealc would have killed him too. But he would angst about it in every intimate scene for the rest of the series. T'ealc's only dealings with other Jaffa would be to replace the leadership with aforementioned hand to hand combat.
Hammond would have been unhappily married and his problems with his wife would have taken up at least four episodes. And he would have been captured and tortured, or subverted, then tortured, or accidentally locked in a closet and tortured by his claustrophobia, in at least six episodes.
Dr. Fraiser would have been sexually active with all the male characters plus Lesbian Replicator Sam. Cassandra would have become a drug addict because of her sluttiness. Then Cassandra would have run amok with her superpowers and Fraiser would have had to put her down. Then, Fraiser would have starred in the greatest SG1 episode ever as she desperately plunges through time in an effort to undo Cassandra's death.
Major Davis would have been an agent of the NID the whole time. All the characters would love having an NID agent around because he had the best one liners in the show. He would have been the hero of at least one episode a year. He would have been the bastard son of President Harold Hayes.
Bill Lee would have been driven mad by his jealousy of Sam's genius. He would have attacked the SGC to take over command of the war against the go'a'uld, believing he was destined to lead humanity to victory. He would be defeated, but never imprisoned, much less killedm although he would keep coming back to the SGC. One or two episodes a year, SG1 would have to cooperate with him in a mission. He would have had a great episode where he hallucinates at random moments that Jack is a go'a'uld.
Siler would have been an alcoholic spy for Bill Lee, until Lee stopped hallucinating but started worshipping the go'a'uld.
Kinsey would have tried to take over the US government but was caught by SG1 and never seen again. Everybody knows that right wing plots against democracy are just schemes by lone nuts.
Jonas Quinn would have kept his superpower of prophecy, and his position on the team. Although his power of prophecy would never be helpful, at least when he was on mission Jackson wouldn't come along and no one would have to listen to that Pollyanna shit.
Get the drift, people.
GOOD WRITING IS ABOUT CHARACTER DEVELOPMENT!