Whatever you do: if you're building cartridges, putting more functions inside 1 cartridge makes it more useful. Shipping costs money, circuit boards cost money, but most parts are cheap. So you might as well cram as much stuff as you can in whatever size cart you pick. For example a combination of (in-cartridge) slot-expansion, 1 MB. or bigger memory mapper & SD card interface (perhaps even SCC or MSX-Music?) would sell like hotcakes if the price is right. You'd probably need a large CPLD or small FPGA to put that in 1 cart... 
I like this idea of integration of various functions. RAM+Sound chip+mass storage in one cart sounds like a great piece of HW....
[SD+megaflash rom scc+psg] sounds like a hell of a combination already :-)
marlon-B, you can also try to contact sinfox, who has developed a nice MSX slot expander with 1024kB built-in:
His email: amanohyo (at) hotmail (dot) com
