My idea is that everyone but one person becomes a sniper with either the stock rifle or the huntsman (maybe a Pyro with a flare gun or a medic with the crusaders crossbow would work too) and the other guy(s) becomes a scout (maybe with increased HP), he has to stay alive for a limited amount of time (lets say 1 minute) while the snipers try to kill him. If the snipers kill the scout, their team and the member who got the final shot get a point (double points if he got a headshot). If the scout survives he gets a point. At the end of the match (maybe after 10 rounds) The team who got the most points wins.
I've made this an unofficial minigame in some skial servers and a lot of people seem to really enjoy it.