Every year, this happens to a few people. There’s no question that it’s good news for them. Not everyone given a second interview late in the process gets accepted, but from what I have seen over the years the percentage is at least 50%, and probably higher. This is a really busy time of year for admissions staff, and it’s a huge commitment for a couple of them (it seems usually to be a couple of them) to take an hour to talk to one applicant. So, first off, you should be really happy about this. You certainly are on the fence, but the vast majority of Harvard applicants never get anywhere near the fence, much less on it.
There’s probably something specific they are looking for from the interview. Rather than come up with a list of points you want to make to them, you would be better served to practice listening carefully and responding. One way or another, they will tell you what it is they want to know – probably pretty clearly, but maybe you will have to figure it out – and you should try to answer their questions, not steer things to your agenda. Your best play is to be yourself, but of course one of the better versions of yourself. Someone they would want to know better, whom they would be proud to have in their community.
Sometimes it seems like they want these interviews because the candidate looks too good to be true on paper, and they want to get a sense of what’s real and what’s fluff. Sometimes, the interview report and recommendations they have don’t completely agree, and they want to figure out why, and which ones are more accurate. Sometimes they don’t know enough about your school or background, and they want to get a first-hand sense of who you are for calibration. Maybe you have some unusual EC or life circumstance. Be prepared to be candid, calm, and, well, attractive. And self-confident, without being obnoxious. You are in the room because you are a substantial candidate on your own terms. So act like that.