That’s a very interesting idea and it leads to the fact that the world building of the premiseverse is so big that it leads to countless ‘what if’ scenarios, like what if Liara refused to assist TIM in retrieving Shepard’s body after the destruction of the first Normandy (would she have genuinely been killed by the AIS leaving Shepard to really wake up in a Cerberus lab with no friendly faces and no chance of a reunion?).

Not that I’m putting any pressure on anyone but it would be interesting to see a one-shot chapter where Shepard does become Rachel’s successor- this could be pulled off by going back to Shepard’s first appearance in this story with the attack on the pirate ship- this time instead of mercy killing the pirate’s daughter she tries futilely to save her with medigel. The mother proves she’s a dirtbag and goes for a gun while Shep’s working and shoots her leaving her so badly wounded she is disqualified from Spectre candidacy. Maybe Shepard could wake up in a hospital bed with Rachel telling her that she’s been so badly wounded that the SA want to dispose of her and uses her dispondancy to woo her into her ‘off the books organisation’.
On a related note, in canon it seemed to me that Anderson presented Shepard as a Spectre candidate only to get her away from Delacor and other manipulations. I’ve long wondered what his true intentions were for putting her name forwards. Due to her damaged mental state(essentially suicidal), controversial reputation and other issues she wasn’t the best candidate and it didn’t look or sound like Anderson was thinking about Shepard’s best interests by putting so much more pressure and exposing her to so much scrutiny the way he did.