The problem with the new day is pretty much the same as the shield had before the split. Together they are hugely over with the crowd but as soon as you split them up, they won't be over because they don'T have a big star between those 3.
The only guy that has potential as a single star at this point is Big e, Kofi is past is prime as a single star and should stay in tag team wrestling and Xavier is pretty much a manager that occasionally wrestles but coulddn'T get over on his own if his life depended on it.
So right now they are better as a unit, but to stay with the subject of the thread, i think the best way to turned them heel wolud be to have to get squash by sheamus and cesaro in their rematch. Then the following day on raw, they have a match against enzo and cass on raw and the lose again pretty quickly and they snap and beat up cass and enzo. So they get into a feud with the most over team on raw and then they cut a promo on the fans and how the hold them down and they have to go back to their cheating ways to actually wins