A specialty ruler *required* for a pattern can be a big turn-off to me. But a pattern that shows how to use a commonly owned ruler is a big plus -- if that makes sense. So it gives me pause to take on a design that requires I spend $30 on yet another specialty ruler. But if I see one that gives you the option of using something like, say, the tri-recs ruler as an option to a paper piecing method, then to me, that's a bonus.
I'm not a big fan of paper piecing. But there is definitely a market for that, and that may be a good niche to look at.