Build your own computer from TTL chips.
You may prototype it with a FPGA implementation.
When I get some intro in VHDL, surely I will :)
VHDL is something that is simple enough to the point I can get a source
in VHDL and understands what it does, but cannot create from scratch :oP
Build new peripherals for old computers. E.g. a QBUS
MSCP disk and tape
controler for VAXen and PDP-11s that uses SD-Cards as backing storage.
I did that for MSX for ages :)