Just read
Sx/Stealth - Bitcoin. It's a pretty cool concept about sending coins without publishing your public key, just your address. Now although this is not something useful for Nextcoin, since publishing your public Nextcoin key increases the difficulty of breaking it from 64-bit to 256-bit, I did get another idea.
It's an idea to hide the total amount of funds in your account from the receiver. In the current Nextcoin system, your funds are concentrated in 1 account, or multiple account which most probably have links in between. Buying something at Ebay would make your account and thus the total amount of funds public to Ebay.
Lets call it a wallet mixer
. Is it possible to make a client which manages several wallets at the same time, and a user interface where all funds are summed up and it just appears to be one account? This way your account could be the sum of for example 20 wallets. If you buy something at ebay, you might just need funds at two wallets and the fund in other wallets will never be known to ebay.