Before answering this recent question about the US motto, I had to check whether the neuter version of plures is plura or pluria. I had recalled right: plura appears to be indeed the sole form used in classical Latin. The same corpus search for pluria gave results of grammarians discussing the choice between plura and pluria. Some seem to defend the form pluria, but I found no attestations of it in a context that does not discuss the very form itself.
So, what's the deal with pluria? Was it a valid alternative spelling, but later all manuscripts were conformed to plura? Is there a good reason to choose plura over pluria? Judging by the same corpus, the genitive is exclusively plurium, not plurum, and this makes me expect pluria insead of plura. All this leaves me somewhat confused.