Numerically Controlled Oscillator would be fun. I just did a design with my Arduino compatible board (see http://tinyurl.com/ydmz2su) but this would be perfect for an FPGA.
Here are a couple of design references.
Snell, John 1988 "Design of a Digital Oscillator That Will Generate up to 256 Low-Distortion Sine Waves in Real Time" Foundations of Computer Music. Cambridge, Mass.: MIT Press
Moore, F. Richard 1988 "Table Lookup Noise for Sinusoidal Digital Oscillators" Foundations of Computer Music. Cambridge, Mass.: MIT Press
(* jcl *)