Ladders in real life, as often as not, stand apart from a wall for much of their length. Sometimes they're propped against an overhang high above them. Sometimes they hang down from a support high above, like on a fire escape. What I'm suggesting is mainly an aesthetic change, but could have some utility as well.
I suggest a few changes to the behavior of vanilla wooden ladders and the addition of a new block, to be described later. Vanilla ladders would no longer have to be placed against a solid block all the time. They just need something to attach to. For the first one you place, this is still the side of a solid block. After that, you can place ladders on top or below the first. They would adopt the original ladder's orientation and rely on it for support. Breaking the original ladder by any means will, naturally, cause the whole structure to collapse.
Ladders could be supported from the top, bottom, or anywhere in the middle, so long as at least one ladder is supported by a solid block. It would be reasonable to limit the extent of the support to 5 blocks, for example, much like scaffolds. But doesn't this make them too much like those scaffolds? No, I think there are still a couple of key differences: 1) ladders still need to be placed one at a time. Thus you may want to just skip them for very temporary construction work, if you don't want to go through that effort; 2) holding shift on a ladder will negate gravity, causing you to remain at your current height. If you really need to keep a steady grip on things, you may want ladders after all.
Finally, I would like to see an iron version of ladders, behaving more or less identically to wooden ladders. This is mainly for aesthetics, once again, so that you could pair wooden ladders with wooden scaffolds/fences and iron ladders with iron scaffolds/fences. Also do note that a fire escape with a wooden ladder more often than not ends badly for all involved.
Tell me what you think. Useless? Useful? Overpowered?