It depends on how deeply you wish to start. For example, the Berlin Defence to the Ruy Lopz (1.e4 e5 2.Nf3 Nc6 3.Bb5 Nf6) wasn’t highly thought of until Kramnik used it to beat Kasparov. Now it is used regularly at the highest levels, and a lot of opening theory around it has been developed. But is this a “chess opening” or just a “variation” of an existing opening?
However I do remember reading a short fictional story about two English players who had qualified for the World Championship Candidates matches (NB This story predated Short and Speelman in 1988, and was possibly written in the 1970’s/early 80’s). In the story one of the players had fashioned the Sokolsky (1.b4) into a formidable weapon, using it in his rise to the top. So maybe this could be a place to start.
