I'm sure there will be redesigns of some sort, nothing too drastic perhaps.
The only drastic change they really have to make is removing the ridiculous face tattoo's. You can cover the body tats with clothing for either character. I don't have a problem with them having body tats in general but the ones they gave them are not the least bit creative and just plain corny to me. Removing face tattoo's might be troublesome story wise I guess but I definitely hope they do it for these characters if they appear in future movies.
As for costumes, just give them something that looks more traditional. With The Joker, it's pretty simple, his traditional purple suit costume is great, distinctive and iconic. I don't see any reason to ever drastically change it, it's one of the best costumes of any comic book character period. I never would have given him the white suit, I hate it, but having him wear it for one movie isn't going to kill me. I'd like for Jared Leto/The Joker to be a large part of the DCU, I think he should appear in 3-4 movies, if that's the case then there is plenty of time to put him back in his classic Purple suit. I hate the silver gangsta grill too but really just ditch the face tats, give him the purple suit and he's good to go.
As for Harley, I'm not one of the people who think her classic costume can't work in live action but I'm also not tied down to it and wouldn't have adapted it straight up either. I can understand why some think the Jester costume is/would look silly. I'm fine with them giving her something else, but this Joker themed trashy street walker look isn't doing it for me. I never would have went away from the red and black, especially for her first appearance, that's one of the more disappointing things about the costume. The pink and blue is something that I just don't care for, maybe for 1 movie, down the line just to change things up, whatever, but definitely not for her first appearance. I think the red and black is a perfect contrast and compliment to Jokers purple suit. I'm sure that seems petty to some people but I think it's great for the character of Harley and the relationship with Joker.
I would have given her an alteration of the Jester costume, taking away the more silly concepts, the headpiece, the white neck/wrist collars, the domino mask, basically making it a red and black jumpsuit type of deal but keeping everything else pretty much intact. For her hair, I would have went with a complete red/black split, not just the ponytails, black eye makeup/bright red lipstick. Maybe some people here wouldn't like that, maybe this is more flashy, less formulaic but that's would have been the best route to go for me.
The Joker and Harley Quinn are both very popular characters, their personalities and blunt craziness are enough to tell us who they are, we didn't need it spelled out all over their face/body and printed on their clothing. Both of the characters are great, both of their costumes were great, they didn't need to be changed, they didn't need to be covered in their catchphrases and turned into human bulletin boards of their characters. Have them do that with their dialogue and actions. I still can't believe they thought these were good ideas. But to each their own, I'll live, it's just disappointing.