Type: 8-bit or 16-bit
CPU: Motorola 68000, Intel 8086, Zilog Z80, MOS 6502, National Semiconductor NS32000, MIPS R1000, IBM PowerPC 601, Intel 486DX2, Acorn ARMv1, RISC-V, Alpha AXP, Zilog Z8000 there are so many processors to choose from. Does it need a co-processor?
How much KB or MB of RAM does your retro computer needs? Does it need banking?
Does it need a separate RAM for music, video?
How many colours would your computer show on screen and how many colours on the computer's palette? How many sprites does it need? Should it include a blitter too?
What sound would it have? PSG (AY-3-8910, SN76489, SID), FM (YM2610, YM2608, YM2151, YM3812, YM2612, etc.) Or PCM (Paula). Would it need MIDI?
What kind of an operating system does it need? Singletasking with DOS-like prompt or a multitasking GUI like Macintosh, Amiga, Windows 3.1?
Any expansion slots?
Media? Floppy disks, CD-ROMs, magnetic tapes, cartridges, memory cards?
Does it need a mascot?