I guess I'd like another requirement on this sub-net too: It should be peer-to-peer and not a centralized solution. Yes, a central server could perform this kind of system with all of the headaches and problems that such a solution also provides. I would personally like to avoid having to turn Bitcoins into a conventional client-server model with a central authority which can be attacked. It goes against the grain of why Bitcoins was originally established.
Bitcoins itself could be built into a central server system too, but I perceive the peer to peer nature of Bitcoins to be an essential strength of the system and something to be modeled onto sub-nets too.
I doubt a peer-to-peer system is appropriate if you have to make thousands of transactions per minute. Because all transactions have to spread to the network. This necessarly takes some time, and I'm not even talking about the process time (hashing of the blocks).
Therefore, cryptocurrencies have an intrinsic limit to the speed it can process transactions.
For very fast transaction, the account/centralised system can be used, as long as it is backed by bitcoins.