Several theories I have about Sage's possible endings:
1) After a time skip in which MC narrates a "Where Are They Now?" epilogue for everyone, he says 'As for me and Sage...' and cut to MC in his first class as a sophomore and Sage introduces herself as the Teacher Assistant for the class. MC expresses dismay that Sage chose to delay her internship or decline any job offers to stay with him, and Sage points out that MC's own mother sacrificed her wealth to follow her heart. MC sheepishly acknowledges this and the two stay together.
2) We get a three year time skip and Sage shows up at MC's graduation, revealing she moved to the Big City for a job, but she and MC have managed to maintain a long distance relationship and can now be together.
3) On a DIK Path, Sage becomes "corrupted," realizing that she isn't ready to give up the party lifestyle quite yet and become an adult, and re-enrolls at B&R as a post grad student and continues as president of the HOTs, enjoying wild booze/sex filled parties with MC and her friends for at least another three years until MC graduates.
4) MC drops out of B&R to follow Sage wherever she goes. Sage is dubious about this, but MC points out his mother did the same thing and it runs in the family.
5) MC and Sage find out that Neil was not the first lover that MC's mother had, and that Sage is his older half sister. Sage commits suicide, leaving MC devastated.
6) On a path where MC slept with Jade, Sage dumps him and MC ends the game alone and miserable.
7) MC sleeps with Quinn's restaurant and Sage dumps him in disgust.
8) MC sleeps with Quinn's restaurant. Sage forgives him, but the game ends with the relationship being very tenuous as Sage no longer trusts MC, and it's a question of when, not if, Sage dumps him.
9) Quinn's prostitution ring becomes public knowledge and Sage loses all of her post graduate opportunities in the fallout, unable to get a marketing job due to the fact she was the unwitting President of a front for a prostitution ring. Neil offers Sage a job on his construction team, and Sage finds fulfillment and satisfaction in blue collar work.