IMHO you've missed the bus by about 15 years, and more like 25 if you want to interface to 8 bit XT bus cards.
8 bit cards are rare like hens-teeth and generally not particularly useful even if you have some. e.g. good working monitors for pre-VGA graphics cards are rare and expensive (except CGA cards with composite TV out), and you'll have a hard time finding 8 bit network cards with even 10base TX - they are usually 10base2 or AUI or worse, something entirely proprietary and non Ethernet compatible.
Considering 16 bit ISA (AT bus) cards:
4:3 aspect ratio VGA monitors are getting harder and harder to find and most old VGA cards cant sensibly drive widescreen monitors. Also the 8:16 bit bus conversion is going to be a real bottleneck - I would expect graphics modes above CGA resolution/bit-depth to be unusably slow, just due to the size of the bitplanes and the convolutions you will have to go through to map them in the Z80 address space.
Z80 <> ISA AT bus for PATA IDE used to be worth doing, but good PATA IDE drives are also getting rare, so you might as well bite the bullet and buy new CF cards and get guaranteed 8 bit mode support.
Ethernet's just about possible, but its going to be a bitch porting the protocol stack to run on a Z80, and when you've done that you wont have enough memory or processing power left to do anything useful. You'd be better off designing a Z80 fast hardware SPI port to access a
WIZnet W5100 Ethernet controller as that has a built-in TCP/IP stack.
ISA sound cards might be fun, but they are likely to require DMA, although if you dust off the *REALLY* old 8 bit ones in your collection, you might find something that's usable without.