Create a piece of (analog? digital?) memory from some form of delay element:

• Reflected Sound
• Reflected Light
• Long Coax
• Sound in a mechanical medium

Use the FPGA to develop memory mapped peripherals for your softcore or existing silicon through an M2M interface.

• Communication Protocols - SPI / I2C / CAN / LIN / etc
• PWM generator

Emulate an aspect of an old-school computer system. Perhaps the NES?

• ROM - Can we load a game into the FPGA and hook it to the NES?
• Sound Generator

http://www.essentialscrap.com/eink/