A little late for this years event, but I wanted to throw in my two cents aswell.
I'm faily new to the shard, so my first time doing the event, and i got a little bored after killing around 400 walkers, because there's nothing more to do and i couldn't see a point where it helps me develop my character further or get a hold on some interesting things afterwards.
The main point i would suggest, even if suggested before, is implementing some quests or storyline in the event, something like for example "rescue a NPC who has himself shut in in a building by killing all walkers inside", "find the teddybear of another NPCs child (by killing the walker/ diving in their old house/just search every crate you find)", "locate the remains/walker/NPC of the husband from another NPC", "provide a group of NPCs with food and/or weapons" and so on....
As for the rewards, deco things like e.g. a blabbering walker head, a arm/hand of a walker that grabs bypassing characters (like spiderwebs), the guts of a walker to decorate a floor or just a weapon to show off (like e.g. a Baseballbat named Lucille) come to mind.
Gold is always welcome

Also like Slymenstrah stated, items that would let you survive a little longer or kill a little easier could be a thing, those could decay after some time and/or disappear upon leaving the event area.
[Glyph]s ideas are also worth mentioning:
[Glyph] wrote:I was thinking that we need more weapon types. Anything can be used as a weapon. No parking signs, trash can lids, a long piece of rebar, etc. These items could have very low durability or have a set number of kills on them before they poof. Possibly have a walker skill that you can raise by killing them. The longer you are in the event, the better you get at killing them but you also lose the defense bonus because of your confidence. How about a group event where everyone goes in as a group and clears an area of town or the woods or even the entire event area. Could be a last day or week thing to help clean the shard. Or even a Team event. How many walkers can Blue team kill in 30 minutes.
Walker skill would reset each year and raise very slowly. Thoughts?
Also it would be nice to have an overall goal, like in the winter event where's those sweet jewel, even though maybe not that powerfull.
All in all, I realy like the idea behind this event, but think there could be something more to it apart from killing walkers
Oh, and a big thank you for your work so far!