If Reigns turns heel he really shouldn't do anything different. Let him continue being smarmy and cocky with his "THE GUY" character. He's already the most over heel on the roster even if he isn't booked that way. I'm pretty sure that he'd muck up any major changes to his persona. Instead, be subtle with it and have him throw in some digs at the "smarks" in the audience (which is really everyone).
I think keeping him roughly the same with only minor differences in presentation and booking would keep the boos coming.
The real problem is the shortage of over babyfaces at the main event level. Rollins is stuck in some kind of nowhere-land because they brought him back as a heel after his injury and now his turn makes no sense. He still seems like the same guy, just now he's teaming with Roman again. Then who else does RAW have? Balor is still on the shelf. They've kind of buried Zayn without giving him a real chance. Cesaro would be great in the main event slot but I don't think they want that.
They turn Roman now, it will be all on Seth.
They could bring Joe up as a face, though it would be kind of sudden. They put him against Roman, though, and he'll be cheered like crazy. Have Roman cut another typical "THE GUY" promo but have Joe come out and tell him he can shove his stupid little moniker up his ass. Better yet, Joe can tell him that he should get a name tag, put "THE GUY" on it, turn it side-way, and then shove it up his ass. The pop would be beautiful.