There is no "fear" of more Commons users; there is (valid) concern that Commons will get a large influx of pictures from one-time users, and that most of these pictures may have to be deleted. We have seen this even for "pre-selected" user groups; when some Wikipedias (Portuguese for one, I vaguely remember) switched to Commons-only, it caused a clash of cultures between Wikipedia-based uploaders on "unsuitable" files and Commons admins.
While better tools would certainly not hurt, the idea of a Commons staging area has been around for a while. But I would like to extend that beyond a simple "selfie filter" for mobile uploads. It could also be used to label, organize, and prepare images before moving them to Commons proper. From Wikipedians-protograph-politicians (German Wikimedia organised this some time ago), over personal photo collections sitting on a hard drive (I have a couplea hundred plat pictures from the local Botanic Garden that need labelling), to GLAM institutions wanting to upload thousands of images, such a site could be a first port of call. It would allow quick, easy upload of files, serve as a filter (admin rights may not be required), allow for batch editing of metadata, and (finally) fully automated upload to Commons.