Would it be
possible to unconfirm people when they get blocked
(possibly as an extra option for admins on the block page)?
Not at the present. You could file a feature request at
bugs.wikimedia.org. But it doesn't seem terribly useful to me (you
have to simmer for four days before making trouble again?).
The reason I suggest it is because the reason most people give for an
edit count requirement is that once you've reached X edits, we'll know
if you're a vandal or not. However, us knowing isn't much good if you
get autoconfirmed anyway. Unless we want to indef block all registered
vandals as their first block, an edit count is useless without it
being possible to prevent autoconfirming if the edits aren't
acceptable ones.