REPONSE A INSOLEUSE

Is the goal to be compatible with the Microsoft Softcard, or is the goal another Z80 based purpose (like the Ramfast or Second Sight)?
16 h 41
If you are running at 25MHz then you can't use just the 14MHz 7MHz signal. 

In short, there's no difference in the slots 1-7 on a II+, //e and IIgs.  There are minor differences (basically, additional signals might be present), but they can be ignored unless you're trying to do something tricky with special clocks or video stuff.  As for a separate crystal--you are in a much more complex design.  The Apple II slots do a lot of work for you with timing signals, and if you stick to the 1MHz Apple II clock, things are simple--this is why simple cards like Mockingboard can have so few chips.  As soon as you add your own crystal, you have to create logic to allow data to cross the boundary.  This isn't "hard", but to do well requires care.  Try to copy a design with its own crystal that's done well, to get the interfacing logic done well.  On a IIgs, the slots run at 1MHz still (again, effectively, advanced users can try to get tricky, but I'm not aware of any products which try to do so).  The Apple does give you a 7MHz clock, if you could live with that, that would simplify things.

It does seem rather limiting that Apple did not implement a 16-bit slot in the IIgs.

The Apple IIgs has 8-bit buses everywhere.  16-bit wouldn't really help, and wouldn't be compatible.

Is there any usage of the 16-bit data bus anywhere in the IIgs?  Surely the RAM must have the upper data lines connected.

There is no 16-bit data bus. The 65816 has only 8 data lines, the typical RAM chips at the time were often 1 bit and installed in groups of 8. (modifié) 
17 h 43
The on board RAM chips for the ROM 01 were only 4 bits wide.