Sometimes you're not quite ready to release a full on version of your open source library to npm. In this lesson, learn how to publish a beta version so people can try it out without tampering with anyone else using your library.
There are a few approaches to this. Most of the time, my beta releases are short lived. I recommend this in general. So you could add this new feature to the beta and hopefully it would be released officially soon.
If you want to have the new feature, but keep the beta version, then you may consider putting your beta versions in a separate branch, then when there's a new regular version, simply rebase the new feature in your branch, and release a beta version of the latest version.
So, in your example, you'd rebase the new feature, then release a 1.5.0-beta.0
But you have quite a bit of flexibility to do things how you want. AFAIK, there is no "standard" or "accepted" way to do this.