Then you would add in a specialization for geography, perhaps class specialization but it would depend on the requirements of the specialization. The localization specialization is always handled separately through the Out Of The Box (OOTB) localization feature (Ruleset specialization).

James hi hope you're doing fine. I have a scenario where I have a proposal case and several subcases. The life cycle of the proposal case is 3 or more months. Now if the class structure has to change where the proposal case becomes a child case for a parent proposal case what is the best design approach for this scenario keeping in mind the existing proposal case and new parent proposal case / proposal case are going to coexist.

Your class structure does not need to change for a certain case type be declared a child case of another case type.  

The same case type can also remain declared as top-level case.  When you want to discontinue allowing that case type of be listed in the New Work menu, simply update your Application rule to say it cannot be created directly.

The only thing that might change is the possible addition of code to the child case to update a possible parent case.  

Your existing proposal case can check for pxCoverInsKey being non-null.  If so, then it is being used as a child case.