I was thinking Bond visiting M's grave wouldn't work anyway cause we wouldn't know the name on the grave stone unless it had 'Head of MI6' on it.
As for a teaser trailer I think it's interesting though.
I don't think they should be adding to many more grave stones anyway. We have Tracey's and Bond's parents. The memento we have for M is the bulldog anyway and if they didn't put her picture in Mallory's office where the one of the old MI6 building is then it may not happen.
I do like the idea of having a painting of her somewhere in MI6 however not any time soon. Perhaps if Dench ever dies they could do it as a dedication to her.