I think that a useful feature in this forum could be the ability to intentionally respond to threads without sending them to the front page and without notifying respondents.
useful in what way? it's a bad idea and should not be implemented on a discussion forum
this will give an opportunity for people to make a few posts secretly to gain activity and other purposes
It can add incentive to discuss older threads without the fear of 'necro bumping' and also when users want to respond to a thread critically without promoting it via bumps.
responding to a very old thread still be consider necro posting and may warrant a deletion if found
how would people know your reply if the thread is not bumped or no notification raised
what incentive would that be if it doesn't encourage any comments or discussions
if needed you can start a new thread and make a reference to old thread to continue the discussion