Anything that would make a fun ASIC could be turned into an FPGA project. Something like Ben Eater's GPU project, which he did on a breadboard, could be implemented on them, or you could make a toy CPU with RISC-V, or create a softcore (faster than a software-simulated, slower than actual hardware) version of any number of older processors.
Edit: Link to Ben's GPU project: https://eater.net/vga