There will be a number of P1 interfaces on the market when the roll-out of smart meters will get into volume. Most of them will be complete with hardware and energy management software either on PC, smartphone or a dedicated display. Quby and Plugwise has already P1 interfacing but indeed closed systems.
It is similar to the devices already on the market to read out the classic energy meters like Enymate or Wattcher. It is not a big business special doing the marketing by yourself and those companies may not survive. A better business model is to promote the product together with an energy provider (Eneco,Nuon,Essent,etc) .
It is nice to have P1 interface hardware with an open API structure ... it may trigger apps development and they will provide all kind of creative energy management applications.
So my specs are
• Interface to the P1 port
• WiFi - so no extra interface is needed on the host site, but it requires key entry
• Hard wire Ethernet (option)
• Zwave module (option may be later) - it requires a zwave interface on the host site but it may be combined with home automation. Don't consider Bluetooth !
• Power supply based on 5V USB
• Storage of data likely 1-4 weeks (in case the host is not on-line and not collecting data in realtime)
You may have a basic version with only WiFi (as cheap as possible) and an extended model with multi interfaces
The first version no storage yet just a read out of the actuale values.
Provide toolkits for popular domotica platforms like Homeseer, Vera, etc
Don't make it too complex and think modular
Also check what is going on on the market - May be the manufacturers of smart meters will come with interfaces see: http://kamstrup.com/media/14059/file.pdf