First off some good ideas I'd wish I'd thought of myself.
Stretch goals for supplements:
Dastardly Deathtraps: Quarentechture as Passive Resistance
Malevolent Maelstroms: Field Observations of Casualty Weather Patterns
Maybe the opportunity to add a quick NPC name/basic concept to a big list in the back of the book, for use in creating on-the-fly NPCs? Since NPCs just need spots and not full character sheets, that could be a pretty easy table to put together. Or an entry in whatever big giant table of random possibilities you'd trust your backers to help with (could save some labor for you, too!).
EDIT: PDFs of printable wailing walls. No hassle in shipping because of PDFs and can be used as props or mood setters.
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Aside from print versions of actual game content/play aids and maybe custom dice at the outside I would strongly recommend against any other physical rewards, myself. They're orthogonal to the point of the project and from what I've seen tend to be overly difficult and expensive to deal with compared to their value as a draw. For my own tastes I'd be going mostly for game PDFs, core rules and adventure content being the biggest draws, but I've chipped in extra for one or two RPPR-related Kickstarters because there was a reward level that contained additional/early Actual Play episodes.
As much as I pitch T-shirt slogan ideas I agree with Crowley's sentiments completely.
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In general I don't back Kickstarter projects. For stretch goals I prefer more content or more of the creator's ideas and thoughts on the project.
For example as much as Game Designer's Workshop has been a FANTASTIC help and really interesting and I'm very glad it's free; GDW is essentially Director's cut commentary for Red Markets and I would pay extra for that as well as "additional/early Actual Play episodes" of Red Markets.
Even the stretch goal "early release of the AP of Caleb's next project TBA" would be a draw I think.
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Stretch goal ideas--Taker name/nickname list -- Ever since I saw the printed list of backer names for use as NPCs in Sine Nomine's Silent Legions, I thought what a great way to fill a need every GM has. As trinite said, allow backers to chose a nickname and have the option of putting their real name as a Taker. For example: Ross "Terrible Monster" Payton. Aaron "Shark Puncher"
--Visual wailing wall -- Gorkamorka's idea of a blank wailing wall got me thinking. Backers could submit a picture of themselves or some personal memorabilia (picture of framed degree, car, whatever) and you hire a graphic artist to make a wailing wall. So you have a useful in game prop with details filled out by backers.
--Become a veteran taker -- In other game lines like White Wolf, before an enemy stat block usually there is a blurb narrated by a character that describes the details of the enemy. A backer who pays a certain amount gets their taker name and choice of voice (soldier, teacher, raider, etc.) to narrate the in game description of an enemy like say one type of aberrant.
--Become a famous victim -- Caleb mentioned there will be unique zombies called aberrants. At this tier the name chosen by the backer is either the in game first recorded victim of the aberrant, or the patient zero who became the aberrant.
goddamn I sound like a sellout. Anyway good luck!
--Caleb takes backer on a tour of a gun show -- ever since the fabled story of the croc-jerky mogul in God's Teeth, Caleb I'm serious offer to take a backer on a tour of a local (to Caleb) gun show. Backer must pay for travel cost.