I started a thread on the English Amiga Board on this topic. I think a classic forum is more appropriate for finding a solution to my question, since I will need quite some help. Here's the link: eab.abime.net/showthread.php?p=1232256#post1232256

@Brian H : Thank you so much for your great answer! It already helped me a lot to understand on which topics I need further reading. I now have a good idea of address decoding logic :-) My ultimate goal is to have the 68060.library loaded from the additional (E)EPROM on the CPU board along with the existing System ROMs, ending up with a Plug&Play edition of the A3660 :-) The goal is to leave the original System ROMs as they are and just add the content of the new ROM on the CPU board (mainly the 68060.library). Although, I don't know if my idea is even implementable...?