I've seen a "conspiracy theory" that the Taj Mahal was a Hindu temple repurposed by Shah Jahan rather than an edifice built by Shah Jahan specifically as a mausoleum. A typical example might be here.
Is there any plausibility to the theory?
No, it is not plausible.
It is undoubtedly (as you suspect) a conspiracy theory, or the sort of revisionist history becoming all too prevalent from Hindu extremists in India lately.
The Taj Mahal was not built back in the mists of time, before reliable records were available. It was built in 1632, which is really not that long ago. There are lots of accounts of Shah Jahan building it and the reasons why it was built. We know who Shah Jahan bought the land from, what he paid for it, who the architect was, when various stages of the construction were completed, how many elephants were used to transport building materials, how much the whole thing cost. There are lots of records.
Even if there were not, the building itself is obviously designed according to the traditions of earlier Mughal architecture and Persian architecture, not Hindu architecture.
The account to which you linked is ridiculous and should be given absolutely no credit.
Source:
http://en.citizendium.org/wiki/Origins_and_architecture_of_the_Taj_Mahal