The most popular missing feature of XCN has always been anonymity. It's been discussed multiple times and there were also specific threads about anonymity and the mini-blockchain.
The theory behind it is pretty complex but there might be easier solutions. Some form of masternode system, or even a simple mixer, might be interesting to investigate.
Another feature which is missing is gui support for withdrawal limits.
What I'm currently doing, besides the endless search for better exchanges, is making the transaction bot, which will be used to test tx throughout and block size adjustment. There might be other uses for it, so I will try to structure it in a way that it can be easily expanded.
If you come to discord, there's a group of people investigating support for smart contracts on the mini-blockchain.
Finally, I'm in private talk for a possible real world usage of cryptonite but it's to early to announce anything.