My decoder desing is almost finished, but I don't know what your instruction set is. You see: my decoder works like this:
fetch instruction : pntcc in which
p=prefix (5th bit for data source selecting)
n=arguments (1\0)
t=type
cc specifies the instruction number
A 4017 counter is used to time everything. After the instruction is stored in the instruction register it increases the program counter. Now if the "n" bit is low it resets the counter. Otherwise the argument is written to the argument register. If the prefix is high it knows that the argument is the data required for the instruction. Otherwise it's an address, so it puts the argument on the address bus and executes the instruction with the data from that memory address.

I built a 4 bit CPU in Minecraft, but I only allowed one cycle operations / instructions, that made it a lot easier. For the decoding I splitted the 12 bit instruction code into a 3 bit OP-code, a 4 bit data part and a 5 bit address part. The OP-code decoder works with an and-gate array selecting the data / address and putting it on the right bus. The special thing about the cpu is the fact that there are two address busses, one for writing and one for reading. That allows fast access of memory and one cycle operations like pushing and loading data from one place to another.

I don't think 12 bits parallel rom/ram exists irl, so you'll just have to use 8bits instead.

No, it doesn't exist yet, but why not try something new? I wanted to make it as narrow as possible to use only one memory cell for every single command.

If you plan making a bigger cpu, maybe use 16 bits. Or a different amount of 8 bit blocks depending on the instruction, but that will make it a bit more complicated.

Most likely I'll be using 1-cycle instructions. Makes life so much easier. And I'll most likely be using 4 bit ram and 8 bit rom.

That's the same what I did, except for 12 bit for the rom, which makes life abit more easier (*MGOS wonders if that's correct english öö) cause the program loads the same speed the cpu cycles. I think you can't do that with any smaller width.

if I really want it I can even just buy 16 bit mem

Then you'd have to shift the data out of the rom, since I don't think 16 bit paralel rom is very cheap, so I'd just go with 8 bit rom.

How many memory locations can you address atm?

I can access 256 mem locs.

Ah, so you have an 8 bit wide address bus. Can you give me your instruction set? How long are your instructions?

I tried using logisim, but I really don't know why they didn't label the pins. I have no idea what to do