As the title says, being able to climb stackable furniture up onto a roof would be nice. Mechanically, it might work like low fence movement, where players (and zombies) climb up the crates.
Example. You have 6 crates with X's on them. You stack them like this:
___ROOF___
X
X X
X X X _____ GROUND _____
You, little survivor human, climb up the boxes to get to the upper level.
Why it isn't OP:
- unlike ladders, nobody's going to walk around with 6 crates or other items sturdy enough and high enough to be climbed
- like sheet ropes, zombies can attack the improvised staircase
- unlike sheet ropes, zombies can clumsily get up a stack of crates
- players who rush up the crates can easily trip, not to mention stacked crates or tables or fridges aren't very sturdy
Why it could be OP:
- getting 6 crates is easier than getting a high carpentry and building a staircase
- zombies might be easier to kill if they're clumsily climbing up the furniture after the player
- if coded in such a way, players could pick up the top X crate in the example above to keep zombies from following, although PZ wouldn't support this at the moment.