Are you seriously asking this question or just having fun?? Bitcoin is a decentralized currency without any controlling authority unlike Banks. Who would keep the collateral? There is no authority who is controlling the circulation of bitcoin!!

However, there are three intangible collaratels available for bitcoin,

1. Public trust
2. Hope
3. Technology

Bitcoin is pretty much based on these three pillars. You can take these as collaterals as well if you really need to factor in. But quantification of these collaratels is not available!