Sitting on a cushion: expand it into a alternate 'use item' method.

Similar to how crouching changes how you use an item, perhaps sitting while using lets you do the exact same thing that crouch/use does, but it could also give you a totally new functionality with some items and blocks. Here are a few example ideas to demonstrate the concept.

* If you like the ambient song playing and you have that disc in a jukebox or portable jukebox, use the block or item while sitting on a cushion to automatically change to that disc (without resetting the song), and it will loop that song until you interrupt it. (And you should be able to get off the cushion and do other stuff, and the jukebox/portable should keep doing what it's supposed to.) 

* Interact with a bundle while sitting on a cushion to reorder items in the bundle however you like.

* Interact with a tamed mob while you're sitting on a cushion to get the mob to associate with that cushion, so it will continue to move around, but will stay in the vicinity of that cushion and not try to follow you when you leave the area. (Bonus ambient behavior: have it then start going to sleep on the cushion at night.)

* If you sit on a cushion and interact using the correct tool with any placed block that has a chiseled or brick variant, you can cycle that block between blank, brick, and chiseled states. This should be with a certain delay time, since you are doing delicate work.