I think your suspecting what if case scenarios, where by mistake someone removes a subnet from one region. Well its possible, but your design should cover those scenarios too.
Often the design is built in such way, assume if EAST pool is down, Users of EAST region are sent to WEST pool automatically, this is done with weightage/score.
Assume if user is coming from x region, but not part of EAST or WEST regions, because for xyz reason its not matching, design has to be built in such way that even if users are coming remaining other regions, they have to be sent to default/fallback pool, you can name whatever you want. Here you can either you NOT condition or use the 0.0.0.0 option.
Try building the infra in your lab and test it. Let us know if you need some inputs.
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.