I don't think that there should have to be any major differences - I'd be completely happy if they kept the "evil" theme from last year.
Well, made it less subtle. I thought that the fishguts thing was too much, and lowered the format to I'm A Celebrity level, in a way.
I think that they started off superbly in BB5 - the small house, bright colours, sharp BB, decreasing prize fund - the atmosphere was great, and would have (and did) lead to some great confrontations (albeit, ones which peaked too early, and had a bad effect on the rest of the show).
I think that if they get the house right, and get the housemates right, then there's not a whole lot they need to change. In fact, if they get it right, they rarely have to do anything extra to the HMs at all..
So, what I'd like to see this year - a more subtle, mind-bending pressure-cooker type environment

Oh, and bring back the eyes on the roof already!