a) Reinette could briefly return in an episode like Dickens did in The Wedding Of River Song.
b) Sophia will either play a companion whose a spatial genetic multiplicity of Reinette or unconnected to the famous mistress.
c) It’s nonsense and Sophia isn’t returning at all and Moffat and company have been playing us.
Either way, we’ll know soon enough, right? Here's hoping because until they actually confirm/announce the identity of the actor/actress playing the new companion, rumours like this one will persist. Personally I'd be open for Sophia Myles being the next companion more than Reinette returning (and I liked Reinette). What does everyone else think?