Don't do a diagonal. The corners in a diagonal aren't really usable space since there's not enough room for a bird to not be smushed up against a wall.I hadn’t thought about diagonal, thanks!
I'd either redo the ladder with about a 45 degree or less incline to spread out the bars, and either have the bars slot in to wall brackets or hinge it to the wall or ceiling so it can lift out of the way for cleaning, or if you don't want a ladder type anymore run a long bar lengthwise though that only gives you room for your current birds.
If you really want to add more chickens, can you expand the coop floor space in the shed?