not sure whether you're serious, but this actually seems like a decent idea. it would require some very well-written legislation to define "perceived severity", though. if someone threatens to kill you, specifically, and you can show that they know where you live, you shouldn't have to wait until they're in your house to do something about it.
Currently you shouldn't use deadly force absent a reasonable fear that it is about to be inflicted on you. But since it is entirely possible to send a threat like I know you live at [address] and I'm coming to [do crimes] and place the recipient in a state of continuous uncertainty, the very open-endedness of the threat manifests the fear of imminence in the target. Rather than making the victim devote resources to continually monitoring and assessing the behavior of the threatening party, we can simply expand the target's right of self-defense in proportion to the scope of the issued threat.
To address the argument, it's not fair to assume that recipients of such threats would be either willing or physically able to respond. People with kids will not go around dueling people in the modern era. In my opinion, it's nothing more than a fantasy world. We have a justice system and law enforcement officers. Maybe they don't do a great job, but it should in no way fall to the individual, no matter how zealous they may be.
The point again is not so much that 'this will solve the problem' as 'threatening people should have a significant cost' and this is one way of significantly increasing that cost without needing to appeal to state actors whose alacrity and integrity may be open to question.
This approach maximizes free speech while providing the targets of objectionable or harassing speech more options for responding to it.
I reject this premise. The social costs for abusing people like this need to be quite a lot higher. Right now it's virtually free and the internet is awash in trolls and malicious behavior. This is a novel situation caused by the confluence of effectively unlimited communication possibilities and we're in no way required to normalize its negative aspects.
One novel approach to rebalancing such costs would be to give the recipient of a credible-seeming threat license to attack the threatening party in proportion to the perceived severity of the threat. So people who credibly threaten others with murder, for example, would be making themselves a legally valid target for homicide.