The goal is 50 slot, i do plan advertising as well though im sure populating the server will be the hardest part in all of this. i wasn't planning on having any mods for world events, heres is a couple examples of how id do a world event with out any mods apart from your run of the mill bukkit.
Project events: Gamemode 1 all participating players, this is already a built in feature to the game that disables upon logging off then back on to the server, preventing players from abusing to much this privilege.
War Events: this one would take some preparation and a bit of seclusion from the server populace but that wont be hard considering the purpous of this event is to clear out new "uncharted" territory. Again, with out any mods apart from basic bukkit, prepare a large area with vacant buildings and mob spawners.
every thing would be discussed in advance and people would yay or nay there participation prior to the event via a forum thread. id probably work it like a world advisory "Forces are approaching (Our world name) they are expected to arrive 1 week from now, will you bare arms and protect our home!???" ( Bam people reply to thread with yes or no, i have a life)