I agree with Steve. Until we get people situated with the server or until we get more dedi's running, we should hold off on major mods like ZS or GG. However, I do like the idea about having "special events". They could make some nice LAN segments or just random days of fun which people could look forward to.
Some kind of map rotation system and RTV kinda thing is good too.
Custom maps will also make it a fun server. Nothing too chaotic though but something that will still be pleasing to casual gamers.
I can't wait to see how the CSS server grows (simply because I suck at TF2 and CSS is the only kind of FPS play style that fits me lol).