the best collateral would be an account, it's easy to transfer the ownership and can be sold quite easily as well, but for 5 btc you need something like legendary with trust or hero with many trust
The best collateral would be an altcoin. I have seen plenty of loan requests using domains as a collateral and most of them are not accepted because the user is offering a shitty domain, if you can prove your site is worth 5btc or more then I'm sure someone will. Why do you need the 5 bitcoins in the first place?
not my site, but my account, i've made 20 btc with this account, so yes it's worth much more than a mere 1 btc
and no, altcoin ar enot a valid collateral they depreciate very easily, and a very unstable, i would not take any altcoin as a collateral not even litecoin
instead an hero account is still worth at least 0.5 even today
Your personal opinion doesn't matter man, stable altcoins are considered as the best form of collateral, you would need to use 110-120% of the value of the loan but anyone would accept altcoins before btc accounts or other type of digital goods, inform yourself better please.