While using a CGI General Leia or recasting would be perilous, this would be a way to add more to Leia’s story beyond what the unused footage contains to help close out her arc, while also honoring the actress by allowing her daughter to act as her stand-in.
The story implications of such a flashback are quite compelling but, just on a surface level, this rumor about Billie Lourd acting as a stand-in in some fashion for Carrie Fisher is cool.