Ok lets see; what and why seems to be enough to explain things. The 'why'part explains what the mod does anyway.
I added a harddisk. Why? Cause I hate floppies.
I added a scandoubler. Why? It enables me to use VGA monitors.
I added extra cooling to the scandoubler. Why? I hope it will last longer.
I added an 68030 accelerator. Why? Cause it allowes me to compile / (de-)comress files faster.
I added 32MB of RAM. Why? Cause it allows me to load lots of stuff at the same time.
I added a compact flash adapter. Why? Too be transfer files between PC/Amiga.
I added a NIC? Why? For networking ofcourse.
I'm going to add my newly acquired 68060 accelerator. Why? See 68030 accelerator and multiply that by 4 (or more?).
I'm going to add extra cooling for the 68060. Why? So it may live longer
I'm going to add a scsi connector. Why? To connect my external CD-Player
That is what I've done / going to do.
There are lots of things you could add (USB for instance). But the challenge of fitting it all inside becomes greater everytime.
This is my scandoubler mod. There are more photo's of my progress there.