If Franz really is Franz. Blofeld, in the books at least, was not only a master of disguise and concealment but of creating seemingly impregnable cover identities for himself. Bond's reaction when he meets the Count De Bleauville in the book OHMSS is to disbelieve that he is Blofeld - at least until he puts two and two together about the contact lenses covering the one outward element of his appearance surgery couldn't alter. Then in the novel You Only Live Twice, 007 muses that "Dr Guntram Shatterhand" may have been a cover Blofeld had been working on for years, for just such an occasion as having to escape from one part of the world to another.
Suppose in this film Blofeld has gone one step further - from creating a new identity to assuming someone else's? Identity theft is quite a big issue these days and is easily done. Hannes and the real Franz might have been his victims - following which, and having sweated out everything from them before killing them, the villain assumes the identity and through alteration the appearance of Franz Oberhauser.
That's one possibility, I suppose. But why would he have done it? Why assume the identity of a man who father was well known to Bond?
Oh well, that's enough potential barking up the wrong tree for now on this.