Seems like there is a DNS issue that i'm not addressing. Since client well be using a URL haven't figured out how to direct clients East or West. I wish i did have access to a lab. Setup some F5's in a VM but that was only good for a 30 day trial license.
If you could post your existing GTM configuration masking sensitive info, we'll be glad to help you frame your new GTM Topo config.
Also, try to see if you already have any existing topology based setups, that should give you an overview of your existing EAST/WEST regions. Because when it comes to regions, its often user-defined and you have to make sure you have all the required subnets. Else you'll have to design in ways that you cover all possibilities.