I find the harshest rejection to be the common one, where a photo is rejected only for the subject being over represented on the database. The level seems to be fairly arbitrary with the photographer having no way of knowing whether his image will be rejected as the 101st or 201st on the db.
As we all use the auto fill (don't we?) what I suggest is that a warning can be instigated on the auto-fill so that when a photographer tries to upload another on a common, a warning comes up with wording such as....
There are already a considerable number of photographs of this aircraft on the database. Please reconsider before submission as the risks of rejection are higher than normal. The quality and aesthetics will need to be exceptional or the photograph depicting a substantial change or incident.
I would not suggest an automatic bar, but a level set by Headscreeners as some aircraft (currently A380s perhaps) would warrant far higher levels than others. It would make all screeners aware when that level had been reached and thus assist a consistent approach to common. It might go some small way to reducing rejections and if the photographer proceeds after the warning, they can have little complaint at a subsequent rejection.