I feel like the series got pretty close to doing rally "right" with GT5, funny enough. That said, I'd be very content to see some more point-to-point venues, perhaps such as the return of the Goodwood Hillclimb, or the ever-teased debut of Pikes Peak. Or maybe even some famous real-world rally stages, like Monte Carlo. At any rate, I still think there are some rally cars that deserve to both debut and get re-introduced, especially for brands without representation in Gr.B. More specifically, for Porsche and Renault respectively. Heck, even the Mitsubishi Lancer Evolution VI rally car would be a fantastic playmate to the Toyota Celica and Subaru Impreza WRX STI from the Group A era, but I digress.
EDIT: Even if the return of the Goodwood Hillclimb and the debut of Pikes Peak were paid DLC like the Power Pack and GT Sport's Lewis Hamilton Experience, I certainly wouldn't mind ponying up, since it'd be a relatively novel experience that wouldn't necessarily "interfere" with stuff like Daily Races, etc.
EDIT2: I'm still hoping for Extra Menus focused on various designers and firms, like Marcello Gandini, Giorgetto Giugiaro, Pininfarina, and so on. On another note, maybe there'll be a Seasonal Menu this June focused on Le Mans racers - perhaps even the LMP1 trio from 2016! Although who knows - if the Seasonal Menus seem to focus on newly-introduced cars, maybe we could even get three Gr.1 challengers at once, especially between the robust list of cars from LMDh/LMH these days. I bet the Toyota TR010 could be one of 'em, as well as the Genesis GMR-001. Unfortunately, I don't foresee the Acura ARX-06 being as likely to be added, now that Honda itself has been seriously considering entering the WEC, but if we eventually could get both, I'd be very content with that!