An instrumental Bond theme? Might not be a bad idea. We could have had one for LTK with Eric Clapton. I imagine one reason we got one for OHMSS was trouble trying to include the title into theme lyrics, although Leslie Bricusse said he was willing to give it a go.
Back in the 1960s it was considered important that the theme tune included the film's title, hence "Thunderball" sung by Tom Jones instead of "Mr Kiss Kiss Bang Bang" performed by either Dionne Warwick or Shirley Bassey. Octopussy broke that taboo.
I said earlier that the Sam Smith rumour was gaining traction, but that doesn't mean it will become reality. He could end up doing it, but on the other hand I wouldn't be at all surprised if a vocal theme for the next Bond is performed by someone few had thought of. Was Chris Cornell on anyone's tabloid media shortlist for the CR theme? Doubt it. Nevertheless, he got the job.