I would suggest to look for the lab of your interest first as it's a very starting age of yours. Then find the research literatures published from that lab particularly. You can then look into some directions of working next. Thanks!
First, you can select your topic by searching what the researchers are doing recently in your particular field of interest. Second, There are enormous topics at molecular level, but you have to decide how much facilities and research infrastructure are available in your lab to accomplish your project.
Zebrafish, fruit flies, frogs, nematodes, mice, or rats are you're main options. They have lots of research reagents which is very important to performing experiments in new fields, or performing in-depth experiments. Personally, I think zebrafish and mice are the most powerful and industry-relevant models to use nowadays due to the speed of zebrafish experiments and the immune system (and other) similarities mice share with humans.
There are a lot of exciting research topics you could work on in this area. But just as stated earlier in the previous comments, you need to consider the previous works of your prospective supervisor and the lab facility at your disposal.