I love that lens. Used to have a 5d mk II and 24-105, fantastic combo. 50 1.8 is such a capable lens as well, and so damn cheap! I used it quite a lot on the 5d, it was more than capable (just don't drop it)
hmmm, maybe some kenko extension tubes (or similar) for macro. Not too much of an investment if you don't end up liking macro.
Night photography is fairly broad
cityscapes/night sky/clubs/nocturnal animals
Maybe a tripod could be your best bet instead of a new lens?
I got a nikon F - EF adapter in the mail yesterday in the hopes of using my E-series Nikkor 50 1.8 instead of buying another Canon one. It's really hard to turn the aperture ring and I can't get it to wide open, won't turn past f/4.
dammit.