Noticed this being mentioned on the dirtsheets this morning
Various news outlets, including Pro Wrestling Insider and WrestleZone, have been reporting on a possibility of an in-ring return for the Undertaker. While to many viewing Wrestlemania 33, it appeared that Undertaker retired but nothing was formally announced. For SummerSlam 2017, it was reported that Undertaker was in the New York area but that may have been something video game related but even that has had conflicting reports.
Reportedly, the Undertaker was working out in the ring before the SummerSlam event and even rumors of him interfering in the Fatal 4 Way match, but obviously that didn't happen.
There is some speculation circulating of some interest in a John Cena vs. Undertaker match in the future, potentially Survivor Series 2017 to continue to assist that longtime Pay Per View (last year featured Goldberg vs. Lesnar and did great business). Otherwise, Wrestlemania 34 is a possibility as a Wrestlemania 33 match-up almost happened but plans were changed.
It should be noted that Undertaker needed full hip replacement surgery in May following his Wrestlemania 33 match (due to lingering injuries before that match, not caused by it).