Well - maybe it will be possible. I hope so. However, one practical point: if the ethernet cart is plugged in, where does the GUI cart go?
This is more like wishful thinking than anything else, but since the Atari ethernet cartridge is still an unfinished project, they might add to the final version the option to boot from a built-in eeprom, I figure that said eeprom should be at least 256k in size to be useful since I am supposing the GUI will be commonly used alongside spartados (which requires 128k ROM) and the GUI+IP stack will most likely exceed 64kb of ROM (specialy if including VBXE extensions). Yeah, I know I am dreamer ^^;