That is a tough one since your dd needs to pick something that interests her.
My interests are microscope projects, plants, soils, insects or animals.
First thing that comes to mind is bacteria. As gross as it sounds, I would like to gauge bacteria under a microscope.
The blood splatter thing sounds really cool.
That's why I told her to find one that suited her. From experience, I know that what what interests me may not interest her. Again, the blood spatter sounds great to me, but I am certain she wouldn't care for it. I thought if someone mentioned something their 9th grader had liked, it might be more appealing to her or at least lead her down a rabbit trail to something she would like.
The idea she and her partner came up with last year was a good one and I have no idea how they decided on it. They did 90% of the work at the partner's house and only worked at our house on their project trifold poster board. When they finished, they wanted to show it to me and I had to keep a straight face while I looked at it and thought, "Oh, crap. That result is the complete opposite of what it should be." Even if I had said that, it was too late to remedy the problem, so I just kept a poker face and let them haul it to school.
When she got an A, you could have knocked me over with a feather, but again, maybe a PP is right and they grade more on the process. I wouldn't have given it an A, but I wasn't the teacher.
BTW, for those who asked, there are no "honors" classes at her high school. They don't use that term. It's on level classes or you take Pre AP for most 9/10 classes and AP for 11/12 classes. They can take a test and get college credit for an AP course. The Pre AP/AP classes earn one more grade point than on level courses. So as a freshman, you take on level Bio or Pre AP Bio. Those are your only choices.
Although her graduating class has technically been dropped from the 4 x 4 requirements, (4 years each of math, science, social studies and English) for many students, the reality is that they will still take 4 x 4. I know DD will. Next year, it's chemistry of whatever level, then physics of whatever level and after that I don't know. I'd take Forensic Science, but that is not her area of interest.
For those who are not Texans, they also must now select an "Endorsement" at the end of 8th grade, which is sort of like declaring a major. It sets in motion what courses, including electives, they will take for the next four years. I don't know a lot of 8th graders who are ready to commit to that, but commit she did. I wonder if any other states have the endorsement? Oh, the fun.
So, has she chosen a science fair project? I have no idea.