It would be nice to reopen my pull request on a different branch when the pull request is declined. For me, the actual problem was the following:

1- I started at <master> branch and created a new feature branch named <fb-some-stuff>
2- I have completed whatever I was doing in the <fb-some-stuff>, so I opened a PR and started a new branch named <fb-other-stuff> to implement other issue
3- I completed <fb-other-stuff> and opened a second PR, but targeting <fb-some-stuff>, to avoid showing it's modification on the diff over master
4- People approved my first PR, so I merged and deleted branch <fb-some-stuff>
5- My second feature branch has been automatically deleted, since <fb-some-stuff> was his target, and I can't reopen it. This means that I'll have to open other pull request targeting master, and all comments in the original PR will be lost.

Other possible solution would be to NOT automatically decline the PR. Show me some options instead, like "The target branch has been deleted. What would you like to do?" with some options such as "DECLINE" and "Retarget".