Ask the developer before you start writing code.
- The feature you need might be implemented already, just not published.
- Or the plugin API changes in the next version, and your code wouldn’t work anymore.
- Maybe the author can just add a hook, so you can implement your code as separate plugin without changing the main plugin.
One of the core ideas of open source is collaboration. Always try to improve together. Avoid publishing something that is 90% a duplicate of an existing plugin. You make the choice for users just harder.
But if you had to write that improvement, and the developer of the original plugin just refuses to work with you, make your code public for the benefit of other users.
The license of plugins hosted on wordpress.org will always allow such alterations or addons.