As noted, WWE has tentatively planned for a John Cena vs. Lars Sullivan match at WrestleMania, however, behind-the-scenes developments this week have caused uncertainty on whether this match will actually take place.
The Wrestling Observer Newsletter reports that Sullivan was actually scheduled to debut on the main roster this week, working dark matches on Raw and Smackdown. However, due to an "anxiety attack", he either didn't show up to Raw or left early. WWE officials made contact with him and "smoothed things over" but the next night at Smackdown, he never showed up to the event at all and ended up flying back to his his home in Colorado.
At this point, nothing is known on if WWE will continue the anticipated monster push that was scheduled. Perhaps there won't be a sign until the Royal Rumble as that is when the angle leading to the WrestleMania match was expected to begin. While WWE is significantly more understanding of mental health issues after their experiences with Mauro Ranallo, it's still up in the air whether the company will remain committed to Sullivan or whether they will decide that it's too risky to invest in him that heavily.