Such feature already exists in image boards like 2chan already implement this feature since long ago. 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.
If you have a fear if necro-bumping a thread you shouldn't be posting in old threads in the first place and if you really do want to discuss something I'm afraid posting on old threads isn't a good solution. Mostly threads that are being necrobumped are mostly megathreads and I'm pretty sure that in those threads several variations of the same answer or two are already existing and all you have to do is read it, if you feel that the answers are still lacking it's better to create your own thread and ask the your question there rather than necro-bumping a thread that has a potential to be spammed by other members. It would really help a lot if your would link/quote the thread you are basing your questions on so that they could be on track with you at the same time.