Cartridges using WAIT and CLK lines?
Thank for the input, so in general WAIT should be ok but it's better not to use the CLK as something that is synchronous to the CPU itself.
@Ducasp, always using the BUSDIR signal for cartridge outgoing signals.
@SdSnatcher73, Working on Paradoxs off-coarse 
I don’t think the cartridge CLK is specified to be the CPU clock. Just look at the Panasonic MSX2+ or turboR computers, it’s always 3.58 MHz.
The only situation where the cartridge slot CLK is not 3.58 MHz is for aftermarket 7 MHz upgrade mods. I guess preserving the 3.58 MHz clock on the cartridge slot is not practical for those, but as a result many modules that rely on CLK (like sound modules) fail to work properly, and the user will switch back to 3.58 MHz for any game or application that has music.
Oh, the 1chipMSX also incorrectly puts the CPU clock on the cartridge slot in turbo mode. Because of this my Yamaha SFG-05 only works in it at normal speed.
The MSX Technical Data Book references the CLK on the slot as the CPU clock, also says the speed because at that moment it was fixed... So, I think that Panasonic took the easy way to keep compatibility with old cartridges that rely on the clock speed being 3.5x.... But since it is meant to be the CPU clock, if the CPU is driven by a faster clock, the correct approach would be to have the faster clock there... At least that is my interpretation, of course, that causes issues because since most (if not all) MSX machines had 3.5x CPU clock, interface designers might have took the assumption route that it would always be like that... 
With all of the clock modifications and slot expanders out there. Is it still save to use the CLK and/or open collector WAIT signal that are available on the cartridge slot?
Are there any known cartridges out there that already have used those signals and still work?
Do you intend to design/build a brand new peripheral that need both signals?
Not necessarily want to use those signals, just looking at possibilities for a new challenge 
