The recycling mechanism.

[fervour]

It would be nice to see recycling implemented in the game, like being able to use old scraps to save money.

Or finding an thermometer from a previous ship and having your kerbal glue it to the side of a ship with some sort of adhesive.

Biodegradable rockets would never get to space though...

[Specialist290]

Welcome to the forums!

You may be pleased to know that with the upcoming 0.24 update, recycling of a sort will be implemented -- you'll be reimbursed for any parts on a craft that you recover on Kerbin's surface, with the exact value depending on how far away from the Space Center you are when the craft is recovered. So in a sense, the game will already implement your idea.

You can find more information on the upcoming update in this FAQ article.

Edited July 17, 2014 by Specialist290

[technicalfool]

As well as the 0.24 features that Spec just mentioned, there is also the Kerbal Attachment System mod. While it's not part of stock, it's definitely a fun mod if you want to build bases linked by pipes, or rip small parts from one craft/wreck and slap it on another.

[DSKY]

It would be cool to have in-game recycling, not just at the meta level. I'm thinking of a shredding/compacting machine that you bulldoze or crane used parts into and they are then deleted or reformed into cubes that can be transported back to Kerbin. Missions could be designed around grabbing debris and hauling it for recycling.