Just-for-Fun has published designs for small MBCs, I ahev three of them: Z80-MBC2, V20-MBC and 68K-MBC.
All with a modern controller for startup and ROM and I/O and SRAM and a real CPU: Z80, V20 and 68008.
All with a SD card and a RTC, all running a variety of Oprarting systems, like CP/M, CP/M-86, CP/M-68K or UCSD PaScal and Basic. All can be programmed with e,g either Turbo Pascal (Z80, V20) or Pascal-MT+ (Z80, V20, 68K).
The user interface is via a serial interface, like USB serial to a PC emulator (Teraterm, Putty, Minicom) or standalone with the Ascii Video Terminal or the FABgl teminal solutions.
Programs to manipulate the binary and hex formatted files of interest for SBC owners. Intel hex, MOS papertape, Motorola S-record, binary, hex conversion fort eh 8 bit world.
Runs on Windows, Linux, Mac due to Lazarus and Freepascal. Source included.