Is it ethical to copy a research idea?
Yes, you can do this. You are not taking his ideas, you are using your own (note you wrote &quot;I have a few ideas&quot;).
Basically, once a paper is published, all the ideas within becomes public knowledge and you are free to build on them. You should still cite the original paper if you are building on their ideas, but your extensions are yours and yours alone.
By the way, many authors will be quite flattered if you work on their ideas - it implies that you find their ideas interesting.