Note: The following is personal opinion off the top of my head, not a demand that things be done a particular way or even a "I'm right and you're wrong."
In terms of having an "official" ongoing source of new material, I'd think the forums here would be the place to field new ideas that have yet to be playtested at all. Guild Companion would be a good spot for ideas that have been playtested among your group and are ready to be crowdsourced in playtesting so to have a fuller base of data from which to draw your conclusions and do fine tweaking.
By the time it reaches HARPer's Bazaar (and as such is under consideration to become part of "standard HARP"), a concept should be so thoroughly vetted as to have no more likelihood of problems and/or errata than the material in the core books, and perhaps even less. A particular idea may not work with the internal logic of your setting and campaign, but you can be pretty sure it works with the current RAW, or if not, with a particular set of already published optional rules that have passed the same testing process.
Of course, that's what I'd like as a GM who likes HARP. On the other hand, it might not be smart in business terms, since your "new" material has already been seen and used by a big chunk of your target market as a result of your playtesting process. Their only reason to buy it now (other than just sheer loyalty) is to have it neatly printed up instead of hen scratch on notebook paper in a binder somewhere, or a folder of spreadsheets on a flash drive.
