As per your question, i predict, you are planning to open a blog. A blog has many posts. And every post can many keywords.

If you write a post over well established keyword then of course it'll show that your site contents are upto date. And it may bring your site upward in latest news or blog search.

If you write about the topics which are less popular then it might impact that you are having some unique posts. And ultimately it'll increase the traffic on your site.

So i'll suggest you that you open a blog. And go for both.

As per your question, I predict, you are planning to open a blog. A blog has many posts. And every post can many keywords.

If you write a post over well-established keyword then, of course, it'll show that your site contents are up to date. And it may bring your site upward in latest news or blog search.

If you write about the topics which are less popular then it might impact that you are having some unique posts. And ultimately it'll increase the traffic on your site.

So I suggest you that you open a blog. And go for both.