Should have a timed shutdown command. Something like "ulx shutdown <minutes>". Access should only be superadmins (must have rcon) and it must require a minute input or give error "You didn't specify time".
I would say it should advertise the countdown every ten minutes, five minutes if under 10, one minutes if under 5, etc. But if you actually had a countdown clock somewhere on the screen, that would be really nifty. Nothing large and threatening, just something simple. The text could start green then switch to red when under a minute. I'll help with the look (at least give my input) if I could just get a coder. Gotta know where we can place the timer first before we can decide where would look the best.