Whatever you design , it should be usable first. Computer network design is not an exception.You are not designing a network to support one service only such as VPN or Internet.It should give enough functionality for today and future needs.
Your design should be usable today and the future !
It was so common to use TDM and Ethernet, IP or MPLS as a transport mechanism at the Access and Aggregation network and for the mobile backhaul. TDM is for the real time , delay , loss, jitter sensitive traffic, Ethernet is for the general data applications.
Building a TDM based network is good for real time applications but expensive at the same time. Thus if you will design a converge transport network, you need to understand your business requirements first.
Which services business want to offer ? What are the trends ?