What elements should a side project have for senior-level web developer?

If one were to invest 3-6 months into improving a CV and applying new knowledge and experience into a demonstrable form which perspectives can be used to determine which path to take to maximize the outcome? For a full stack web developer.

A few examples:

- search ads for the ideal job and study their requirements closely and try to cover them all as much as possible

- build a small Saas, think of original idea, and try to get a few real world customers, so you can say "I build real things"

- add one algorithms, system design feature, they are very favorable in interview process of big tech companies, like in this example:

https://blog.pragmaticengineer.com/an-educational-side-project/

- try to make something reusable for other developers, library, starter project...

- build something visually impressive, rich UI and CSS, 3D graphics (WebGL)

- try to go wide and cover all parts of the stack

I know there is no universal answer but you can start from yourself, what thinking is closest to you if you were to build it, or what would you like to see and what would impress you?