🚀 go-pugleaf

 An S-100 cage will be very big. Very, very big. That's OK if you really want
to use all of that volume for electronics. However, nowadays that takes a lot
of parts. When the S-100 was first popular it took a whole board just for the
processor & support - another for 16K or RAM, etc. A stuffed card cage would
have you running a 64K 8080 system with (gasp!) a printer and (wow!) a TVT
and matbe you could have room for one card of experimentation real estate.

 The S-100 decreased in popularity because of its physical size (both the
enclosure and the cards themselves - no manufacturer wants to make anything
bigger than they have to) and the obsolescence of the backplane standard.
The S-100 signals are just TTL-level with some passive termination at one end.
There was bitching about the S-100 crosstalk even in the days of 4MHz Z-80
systems - they were just too darn fast. With edge rate control you can push
the performance into the low 10's but the S-100 backplane is pretty 
unsophisticated and I'd expect you to see some flaky performance if you try to
go full speed (like 25MHz). I have a 68000 based S-100 (8MHz) system in the
attic. It measures 2' by 2' ( terminal not included ) and weighs in at about
80 lbs.  

 If you want a smaller card cage standard with higher capability I'd say the
VME is one to look at. There is a high number of pins, good availability of
enclosures and function, processor and memory cards, and the bus seems to
be capable of supporting higher-speed systems. The drawback to VME as I see
it is that it is a commercial standard, not a hobby standard, and you can
therefore expect to pay commercial, not hobby, prices for your hardware. The
blank boards and card cages are only moderately expensive, but something like
PC capability costs like a PC or maybe 2X. You end up paying for compactness
on this end. I also have a dual-processor VME system in the attic. The cage
is 6" by 8" by 2'. It is unenclosed though. I'm not sure whether the signal
standard is TTL or some other line X/R thing. Since you plan to assign your
own signals you may have some trace cutting or component removal to do if
you start with a populated backplane. The bus standard may not be what you
want in some cases.

 Then again, the venerable STD bus (44-pin) still has a lot of activity, mainly
in industrial control type stuff. If you can get by with the low
bus width you at least can get the proto cards down at Radio Shaft. 

 PC-bus cards are available which insert various other processors into a PC
system. Prototyping cards are also available. There may be manufacturers of
PC bus-extender chassis' out there - I vaguely recall seeing that kind of
thing; they usually appear after the product has been out long enough for the
hardware freex to get edgy but not long enough for the next generation or
clones to arrive. 

 Probably a good place to sort this out would be a PC magazine and/or Byte.
any of those are 80+% ads. Some of them will have to appeal to you. The Byte
issues for December of each year used to have index sections for the year's
articles - you can probably find stuff about the various buses in there, in
more detail and religious fervor than you may want.