With "designing cartridges", you mean "design hardware that plugs into an MSX cartridge slot" or "create software that runs in a cartridge"
In case of the latter: use a Flash EPROM, and wire that with a mapper mechanism so you can access all locations. Then use some software to re-write that Flash EPROM. Easier yet: get a Mega Flash ROM SCC+ from the MSX Cartridge Shop.
If it's just for software design: consider using an emulator for development. They're very good these days, and built-in debugging facilities can be very helpful.
I'll second the recommendation for the MegaFlashROM; combined with a good emulator/debugger (BlueMSX is quite nice,) you couldn't ask for a nicer development platform.