If B is a spiritual successor to A, then what would one call A in relation to B? The obvious answer would be spiritual predecessor, but that seems inaccurate, since spirit seems to refer to intent - B was intended as a successor, but A is not now intended as a predecessor in retrospect. Are there any alternatives?
I think forerunner fits nicely.
Forerunner - a person or thing that precedes the coming or development of someone or something else:
The icebox was a forerunner of today’s refrigerator.
Listed synonyms include: predecessor, precursor, antecedent, ancestor
Actually, "spiritual predecessor" has been used. See here: http://en.wikipedia.org/wiki/Supreme_Commander_(game)
and further down the page:
If you don't like that, I think inspiration would also work:
I'd say that a spiritual predecessor is not the same as a forerunner. To me, "forerunner" implies a tighter connection. In this case if "Total Annihilation" were the forerunner, I'd expect some sort of story continuity between the two games. And having played both of these games, I'd say that "spiritual predecessor" or "inspiration" are the more correct terms in this case than forerunner.