Enterprise WAN Routing Design

How would you connect enterprise offices and DC today. What are some options – Physical + Logical – and then how would you do IGP? Will you do OSPF and if you do how would you go about areas? All branches have local upstreams for example.

Or is it now norm that everyone is doing private ASN and what have you and BGP between sites. What if your edge routers already have same ASN and BGP for upstream how will do BGP between sites and be forced to do a full mesh or RR setup.

When designing for a new (non-MPLS) internal WAN which will be dual stack sooner or later, what approach should one take regarding IGP selection and design? Is it generally better to have one IGP process which supports both address families? Which design choices should be made at the onset of the project to minimize design changes if v6 is added later?

It still seems the “default” IGP these days is OSPFv2 which only supports the IPv4 address family. Even if v4 is used on day one, should one consider OSPFv3, IS-IS or MP-BGP which has multifamily address support?