|I'm sure someone can correct me if i'm wrong but:|
The 68000 CPU has a single address space of 16MB
The 60020 CPU added an additional address space of 4BG
(68EC0020, like in the CD32 only has original 16MB space)
To be compatible with the 6800x0 line the Apollo core must support both the 16MB and 4GB address spaces.
Keep in mind that everything must have an address: ie., ROM, chip RAM, system RAM, expansion boards, etc...
Also, moving the ROM, or chip RAM out of the original 16MB of space would have compatibility issues. I'm not sure how severe.
Given the situation i think the best solution is the original 16MB contains 8MB of chip RAM and other needed items like the ROM. Then the other 4BG of space can be divided as 2BG for fast RAM (the AmigaOS can't allocate more then 2BG for any one application) and the remaining 2BG or expansion boards like Zorro cards.
Another very large addressing block could be added to the Apollo core. However, this would require both work on the Apollo core and AmigaOS would require a major update.