Since people are adding on ideas, here is a thought I had. Not really sure where I'm going with it but it something feels right about it.
In the future we can't really be sure what NEM will evolve into. In fact if it is successful it will actually look quite different then it does today.
To that extent I think it would be interesting and possibly useful to reserve a subset of account numbers for later use so that in the future these can be specialized.
I was thinking for instance that when we made Test NEM accounts they all start with "T" and our genesis block addresses always start with a subset of letters. I was thinking maybe it would be useful to reserve all addresses that start with some letters like NSxxx-xxxx-xxxx-xxxx to be reserved for later.
Again. Not sure if it's a good idea or not. Just thought I'd mention it.
Isnt it enough for future uses to have another letter than N in the front? 
If it is possible that would work too 