Well as said, I fully accept the, er, 'risk' of being pleasantly suprised, it can and does happen as we all find it very easy to defend an old film when a role is being given to someone else - But the fact still remains, Mel has always been Mad Max and will always be in the mind of fans of the older films.
Regarding the Dark Knight, a friend of mine put it like this (As an avid fan of the Joker and someone who originally hated the idea of HLedger as the new Joker):
"Ledger wasn't a great Joker... He was a great antagonist, hell yes, but it wasn't the best Joker". People will always have their view muddied by preference or opinion, especially when you only have one (or a few) previous examples to go by, like Jack Nicholson vs Heath Ledger, or now Mel v THardy. For the record, off-topic as it may be, I enjoyed both Jokers very well and respected that each one had to be portrayed in a manner that suited the tone/theme/settings of the movie. JN's Joker would not fit in TDK much like Ledgers wouldn't have in the older Batmans, for example.
Ultimately I hope I'll be pleasantly suprised but as said, it will suck if Mel doesn't even have any form of Cameo - Almost every movie does this, even Charlton Heston appeared in the newer Planet of the Apes (Ironically as an APE!), just as a mark of respect if not for improved interest.