I imagine a scenario similar to that one. Undertaker either wins or loses his last match at Wrestlemania, and then the next night, they do some sort of special effects shit (kinda like Royal Rumble 1994) that symbolizes Undertaker finally 'resting in peace' for good. Maybe he disappears into the air or he gets put in a casket and carried out or something imaginative along those lines. This would work better if he lost the streak considering it'd be the ultimate end for Taker, but I don't see that happening. Either way, once Taker retires this way, I don't think he should ever appear on WWE TV again (except for his eventual HOF induction), just to keep it up with the whole mystique of the Taker character.