Inversion of Spiritual Successor

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)

Supreme Commander ...is considered to be a spiritual successor to Taylor's 1997 game Total Annihilation

and further down the page:

The Supreme Commander Official Soundtrack is the musical score to Supreme Commander. It was composed by Jeremy Soule, who also composed the score for the game's spiritual predecessor, Total Annihilation.

If you don't like that, I think inspiration would also work:

It was composed by Jeremy Soule, who also composed the score for the game's inspiration, Total Annihilation.

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.