The best practice currently is to obtain current copyright holder's permission - a license to use the copyrighted motives; possibly, for a fee or percentage in expected royalties.
This is not always viable or possible. In this case you may either change enough that it doesn't violate copyright (but becomes a different story entirely) or pick more guerrilla solutions.
Going about commercial release is no longer viable. Even if you turn profit, you may expect a cease&desist followed by a lawsuit, and while the copyright holder is not guaranteed to win, your outlook isn't very good. Plus you antagonize the side of author on which you write, not a good marketing move.
Much more viable option is simply entering the fanfiction realm. You release for free, anonymously, and even if it's taken down on one site, you can reupload it on another... while fanfiction is "grey area" with law on the side of the original author, the "cruel reality" is that the author and the law lack technical means from preventing even most unwanted fanfiction from cropping up.
Of course this way you won't make any money, but essentially the moment the author said "no, I won't issue you any license" you lost any chance for them anyway.