When is a new method publishable?
In general, for any scientific paper, the key question for being able to publish in a reputable venue is: "Do we learn something new and interesting?"

In this case, I think the key questions to ask in this regard are:


Is there a set of real-world (or otherwise interesting) problems that fit the more general case of algorithm D, and for which A/B/C are not applicable (or perform badly)?
Does algorithm D also out-perform even-more-general algorithms (e.g., some of the foundational algorithms), or do they not apply for some reason?


If the answer to both question is yes, then you've probably got a nicely publishable result.  If the answer to either one is no, then you've probably got something new but not particularly interesting: that may still be publishable in some venue, but probably not well and probably won't have an impact.