Common ideas are unlikely to be protected from competition by an NDA and constrain the other person's actions in a fairly significant way. Uncommon ideas are innately protected by requiring consecutive insights in a subject area which others are highly unlikely (in the sense that if you require 15 ideas that someone might or might not have from a subject area, there's a three in a hundred thousand chance of whoever someone talks to about it having that overlap) to be grocked and spread to someone else who's going to leverage it.
In either case it's difficult to see why someone would sign an NDA, that is largely to your benefit and the exclusion of theirs. Asking them to do so is roughly equivalent to the claim you're about to show them a unicorn; a simple, effective idea that somehow the rest of humanity has missed.
