Homebrew on the Giz has just been cracked. Give it a 4" widescreen display and with the power it's got under it's bonnet, it's a much much sexier beast than it was at launch.
There're two games I'd play megaCD emulators for : Sonic CD (of course) and Switch. I've always wanted to play Switch but the action replay cart I had wouldn't allow me to play it (damn sega, upgrading their region protection!). I seem to Remember the Mega CD have a couple of decent shooters and a quasi-3D chopper game which was quite fun (thunderhawk?)