this would first prevent the "signature campaign hopping" and on the other side, would be here in this forum more user with a signature on the way

Funny that you would say that given that you are not wearing any signature and applying.
I guess most quality posters will have a campaign already. So it's likely one of them will be chosen, and not someone not wearing any.
how about the suggestion, that this time the applicants who are not participating in any signature campaign at the moment will be selected/accepted?
this would first prevent the "signature campaign hopping" and on the other side, would be here in this forum more user with a signature on the way

in OP
Note2: Due to the enormous amount of applications even when there is only one open slot, changing your signature to apply is not required.
I think you completely missed the point.