#4. Saving him for the Royal Rumble

What better way to have an impactful debut than by entering the Royal Rumble? He can't exactly be in the coveted #30 spot because R-Truth won the right to that. However, he still could take that if Truth is assaulted by a mystery attacker in a backstage segment during the Rumble.
Sullivan being at #30 would be as impactful as you could imagine, and if Cena does make it to the Rumble, then all the better! Sullivan doesn't need to win the whole thing to make a statement.
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#3. Debuting on the RAW after Royal Rumble

While it may somewhat contradict the last point, look at Samoa Joe two years ago. Fans wanted him to debut at #30 at the Royal Rumble but were angry, frustrated and disappointed at Roman Reigns coming out in that spot instead.
However, Samoa Joe would debut the next night in the main event segment, creating an impact, not having to be eliminated and getting his own time to shine. You can definitely expect Sullivan to debut at the RAW after Royal Rumble at latest.