Moses 4:1 describes Satan's role in the council in heaven:
"And I, the Lord God, spake unto Moses, saying: That Satan, whom thou hast commanded in the name of mine Only Begotten, is the same which was from the beginning, and he came before me, saying—Behold, here am I, send me, I will be thy son, and I will redeem all mankind, that one soul shall not be lost, and surely I will do it; wherefore give me thine honor."
In verse 3, God explains that Satan was cast down because he rebelled, sought to destroy man's agency, and sought God's own power.
Under the traditional theory of Satan's plan, this verse is a little confusing. Why would Satan say "I will be thy son"? And why would he offer to "redeem all mankind"? If he would not allow mankind to sin, why would we need to be redeemed?
I believe Satan was offering to be Heavenly Father's Only Begotten Son and suffer for our sins, just like Christ. In fact, earlier in Moses' vision, Satan tempted him and tried to get Moses to worship him. When Moses cast Satan out in Christ's name, "Satan cried with a loud voice, and ranted upon the earth, and commanded, saying: I am the Only Begotten, worship me."
Satan is a deceiver, but he is consistent. He wanted to be the Only Begotten in the premortal world, and he tried to convince Moses he was the Only Begotten in the mortal world.
Under Heavenly Father's plan, Jesus Christ would suffer for the sins of all mankind. His atonement is infinite and eternal, which I understand to mean not limited to a certain number of sins, but somehow encompassing all sin. All of our sins have been paid for, whether or not we repent. But because Heavenly Father seeks to build us up and to have us return to him, He chooses to forgive only those who repent. This is the only way any of Heavenly Father's children will return to Him. Satan and the billions of rebellious souls like him didn't like that idea. I imagine that Satan's argument may have gone something like this: "If all of our sins are paid for, why should anyone suffer for sin? I will go. I will be the redeemer. But I will forgive everyone. No one will suffer for sin. There will be no punishment."
Satan thought to "save" us (or at least himself and those who followed him) by destroying the consequences of sin. At best, his plan may have provided for a resurrection and possibly some degree of glory. However, his plan would have destroyed our agency, destroyed our chances of returning to live with Heavenly Father, and ultimately was inconsistent with the very existence of Heavenly Father. Because of Heavenly Father's character and purpose for us, he could not even consider Satan's plan. This theory of Satan's plan is not only more consistent with Moses 4:1, but it is also supported by scriptures that discuss how Satan deals with mankind. Under Satan's plan, there would have been no consequences for sin, and that is still his message today.
In conclusion, based on the present scriptural record, it is more likely that Satan offered to be a savior by suffering for our sins and forgiving all mankind than it is that he offered to save us by forcing us to do good. But I doubt Satan would have been able to follow through with his plan. We know that Christ suffered terrible anguish in Gethsemane for our sins because he loved us. I can imagine Satan feeling a part of that anguish, deciding it was too much and that we were not worth it, and getting up and walking away, while we all looked on in despair.
I recently put together a brief description of what I think Satan's plan was for us, based on the scant scriptural account. The above thoughts are an excerpt of the document, and the full document may be found here. Although it is a finished thought, it could probably still use some fleshing out. If anyone has comments, citations, or arguments in support of or against this idea, I would appreciate them.