For lifelessness:
* lifeless
* inanimate
* dormant
* extinct
* unresponsive
* still
For lack of activity:
* stagnant
* idle
* inactive
* quiescent
For complete cessation:
* gone
* finished
* over
* spent
For extreme sadness or despair:
* broken
* devastated
* heartbroken
* crushed
For complete silence:
* silent
* mute
* quiet
* soundless
It's also important to consider the tone you want to create. "As dead as a doornail" is more informal and humorous, while "as dead as a stone" is more somber and serious.
Ultimately, the best replacement for "as dead as..." will depend on the specific context and your desired effect.