I don't mind the current regs or the R5s but the problem with them is there isn't enough disparity with the body styles. Almost all S2000-based rally cars and the R5s are 3 door superminis. The only exceptions I can think of are the Fabia S2000, a 5 door supermini and the Toyota Corolla & Auris S2000s - 5 door compact (or whatever one size up from supermini is) hatchbacks. If I have forgotten any please chime in :D
I don't know if there is any way regulations could successfully encourage use of more body styles but surely increasing minimum size would help as otherwise the manufacturers are limited pretty much to only using superminis.
In an ideal dream-world I'd have something like 2011 WRC but with similar measures to cut cost as R5 but the body style open to 'prototypes'. By that I mean Ford may enter the championship with something looking very Ford-like but they can sex-up the body shape instead of making it look like a Fiesta. Toyota could make a car looking distinctly Toyota-ish with a 3 door coupe body and call it, for example, 'Celica GT-Four' even though the road car doesn't exist.
The cars would be interesting to the fans because they're not limited to whatever boring crap the manufacturer sells but they'd have the VW, Ford, Citroen, Hyundai, Toyota etc. 'family face'. It's like Group-B for modern times. I personally hate mentioning Group-B because it's silly to live in the past, but I think having a 'prototype' rally car is a fairly simple (please prove me wrong :D) way of making spectacular looking rally cars - without resorting to £250,000 homologation specials with too much power to be safely hurtling through a forest or alpine road with and fuel tanks mounted under the occupants and magnesium everywhere to light up in a ball of flames!
Dream world I know but that's just my two-pennies worth!