1) Corporeal beast mass with increased rates for sigils / Potentially shortened respawn timer for faster kills (Halved sigil drop rate maybe?)
2) Ability to use the pvp arena for tournaments resulting in item rewards/store credit (Think DH tournaments with the deadman-style arenas)
3) Having an increase to the xp bonus well gives for duration of event - Mabye 50% for 4 hours to differentiate from a normal player-funded well
4) Event idea - Double drops server-wide, Hosted for an hour most likely - could require tweaking of certain drop tables (whip,trident,dboots) Maybe a rarity threshold like 1/150?
5) Buffed wintertodt crates for event - maybe achieved via an increased point reward per action during event?