Abstract the interface for TmtDealPosts so that it can be used to create different post types (I guess that now it's not much more than a shell for registering a post type).
Extend tmtDealPosts plug-in to create both "Deal" and "Coupon" post types. "Deal" posts are already in use as the primary content type. "Coupons" will contain narrower coupon information, and I expect themes will actually pull "coupons" into "deal" posts.