First, I do think Dolph Ziggler could make a great face. In fact, there's a good chance in this writer's opinion, that he could be THE top face at some point in the company's future... But for now he's the best heel in pro wrestling, followed closely by Wade Barrett, the Miz, Jeff Jarrett, and Bobby Roode. I don't think they need to be throwing away that precious commodity so soon, especially when he has a lot of territory left to explore!
Best thing they can do for Dolph Ziggler is push him to the very top as a heel, get the fans behind him with solid heat, legitimate his place as a main eventer, maybe a few World or WWE title reigns... and THEN you turn him. Doing it any time before then is just premature, and could result in a burn-out much too quickly for a rise star of his caliber.
Build up Ziggler with a Royal Rumble win, or if not stay AWAY from the Money in the Bank. It's a great concept, but he doesn't need it to get over. If he wins the Rumble, pit him against the champion at WrestleMania. Maybe not this year, but next year at the latest. Give him that big WrestleMania title win and put him over a huge face like CM Punk, John Cena, or Randy Orton to really give him the necessary heat.
How to actually turn him though? I've always been a big fan of bringing a guy as close to the edge as possible, and seeing what it takes to finally make him snap. In this case, he'd need to not be the champion, but possible the second biggest heel on the brand. Somebody who does the dirty work, maybe under a guy like Wade Barrett or a newly turned Sheamus. They'd want to have the bigger heel go on some sort of rampage against smaller, weaker victims to solidify their run as a champion. Very similar to what Sheamus did in his first run, but WAY more brutal. I'm talking destroy Rey Mysterio, bashing him into the ring post a dozen or so times. End somebody's career, and put a few out with injury. Get involved with matches you don't belong in, just because you can... That type of heel. I'm thinking Triple H as the head of Evolution type heel. Somebody the entire crowd can hate unanimously! In fact, some sort of Barrett, Ziggler stable might do the trick here...
Then you're going to want to have the head, we'll say Barrett train up Ziggler to do the same. Have him start interfering with matches, eventually taking it up a notch and participating in the beatdown. Eventually Barrett will ask him to end somebody's career; take them out permanently. Maybe somebody like Cena that for some reason comes to Ziggler's aid a month or so before. I don't know...I'm just thinking out loud here guys! In the end Ziggler is going to need to be pushed to the very limit, and not be able to follow through. Barrett would come out and finish the job for him, taking the face to his knees and destroying him within an inch of his life. Blood would be preferable, but that's likely not going to happen... And it's just as simple as Ziggler turning on Barrett in the end, saving the face from the final blow. He attacks Barrett and in the process turns face. The two come to blows for the title at the next PPV, and Ziggler gets the big title win and is no a full-fledged face champion.