Help! What else can I add to my essay?

Sounds like you may be repeating the word "serendipity" rather a lot in this essay. Although, seriously speaking, it also sounds like you've covered a lot of good ground.

I can't think of any topics or new aspects you should cover as such, and it's hard to say what could be expanded or improved without reading your text. Just make sure you're not only talking about serendipity but contrasting it with other things, defining it clearly and bringing it nicely into relief for the reader. Just add stuff about how science normally works. Maybe don't jump directly into what scientists have said about serendipity but add an introductory paragraph where you delineate what it is against how science normally works and present one good example of it.

Not a super helpful answer, but best of luck. You'll squeeze another page out it somehow.

where serendipity's role in science is going in the future, possible scenarios involving serendipity in science