If Bitcoin Talk introduced a shop, and one of the items for "sale" was the right to display a signature, then signatures could become a privilege that had to be earned through responsible posting.
Wearing signature is already a priviledge which is earned through quality posting, not sure what responsible posting is.
The current merit system means to wear a signature a member has to earn at least one merit, and to get to higher ranks and unlock more perks, different signature style, avatar etc, more merits has to be earned.
So the higher you go, the more work you have to put in, and gladly this is put into consideration as most bounties pay higer ranked members more.
The system is not perfect, but neither are your suggestions.