It really should not be 6.0, since M is still classed as an incremental update.
Lollipop was to Kit Kat just as ICS was to Gingerbread in terms of those being major updates (ICS at 4.0 and Lollipop at 5.0)
I'm hoping that Android M should be 5.5 but I might be totally wrong, let's wait and see.