Since other filters don't cause security problem, I don't understand how a new filter could cause security troubles (but I'm not developer)...
In fact, I was thinking at some filters:
¤¤|http://www.dailymotion.com/ -> would put a placeholder instead of images, frames, objects on dailymotion
¤¤$third-party,object -> would put a placeholder for all objects of third-party domain.
These kind of filters (¤¤ in my example) would allow to have "blocked elements" (difficult to unblock, but that's what wanted) and also elements which could be unblocked in one click.
For the "immediate unblocking" problem, I don't really see the problem since Noscript and Flashlock manage to unblock element after having clicked on placeholder.
I really think placeholder is the way to go in order to have a better way of blocking/unblocking element...
For now, in order to have placeholder, we have to use NoScript or Flashblock, and both are not very compatible with AB+.