I don't think that there is anything wrong with your proposal. But for really estimating it, it is not concrete enough. It would be helpful to start the 'further discussion' on detailed policies such as ranges for r.
I have not thought about that really. Quite frankly there is a lot of missing information before one can get that concrete. The most important piece missing is information on interdomain (exterior) routing schemes to be used. That's why I pointed out Mike O'Dell's 8+8 draft. Before routing becomes more clear any address allocation/assignment scheme needs to be very conservative in order not to preclude too many options in the high order bits. It also has to have the label "preliminary, provisional" because it may otherwise become useless.
That's a good point. In fact, we are looking for a scalable and efficient (exterior) routing scheme. Given that in the future there will be MANY ISPs worldwide the vast amount of associations of provider-based prefixes to ASes might impose a problem to the size of the routing tables. So, there is some need to aggregate routes to providers. Having regional prefixes is one option.
In the light of this and the fact tht we are not exactly overwhelmed by ISPs asking for IPv6 address space I doubt whether we need to discuss concrete schemes at this point. However we should keep this item on the agenda and have a discussion at the January RIPE meeting. We should also watch developments at the upcoming IETF.
Yes, be prepared :-) Frank