The more I think about it, the more I realize that Richard isn't going to make it past the season finale. He could never come to work for Nucky because Nucky killed Jimmy. I know that Richard told Nucky that he was 'safe', but it would still be way out of character for him to actually join up under his employment.
I guess finally he could join up with Capone. This is really the only situation I see as an actual possibility. Capone and Jimmy go way back, as do Richard's ties with Chicago.
Or maybe he rides off into the sunset with Julia and all is forgotten. But again this just wouldn't fit the character. Richard has led a tragic life and it sadly only seems fitting that he goes out in a tragic manner.
So there it is, unfortunately I think we will be seeing the last of the masked man Mr. Harrow.