Choosing IP Schema for Single Domain - Clues!!

["WALI" <hkhasgiwale () gmail com>]

Some time ago, The decision was taken by my corporation to shift to Single 
Domain AD model rather than the current multiple, parent child domains 
spanned across various geographical locations connected via VSAT/leased 
lines currently.

What will precede Single domain would be a Global WAN where one service 
provider will likely manage MPLS links across various office sites. The
dedicated bandwidth will vary from 512K to 2mbps.

My recent concern is to think about designing an IP Schema for all locations 
spanning across UK/Gulf/Asian countries. Some IT guys are of the opinion 
that we would assign IP addresses from the current pool at each location to 
the new servers and undertake migration. I feel that re-designing the whole 
IP Schema would be a better idea because the current one is very haphazard 
and and has evolved over time.

While I push for a new design, I need to take of security issues and future 
scalability at each location.
What all factors should I incorporate in my IP design in this scenario? Is 
there anything I have to take into consideration vis a vis MPLS backbone? 
Does anyone has any experience in designing an IP schema for such a project?

All advise is welcome.