Wrestling being what it is, and especially the way the WWE moves, I think a fitting end would be one last good match. Maybe Taker will know when that match will be and there's an angle done around it, ie a retirement stipulation or simply just his saying this match is his swan song. Wrestlers being who they are, it's hard for a wrestler to say so short of having a career ending condition like Edge suffered. The last couple years, the product has moved on without Taker. He's still a big Wrestlemania attraction, but wrestling and the WWE has moved on.
If Taker continues to do matches without being certain if this is his last, he could very wait six months after Mania, try training and decide his body doesn't have another match in it. Fans might like to see that big farewell show, and with RAW being 3 hours, they could probably use any sort of programming ideas.
But Taker has long been one of those private guys that doesn't speak as himself on camera very often. Even when he did the American Bad Ass gimmick that let him speak a lot more freely, that was still very worked.
I could very well see a guy that doesn't mark out for himself coming to a decision a few months after his last match and saying he's done and wants his departure to be quiet. The Undertaker had his time, and when he decides he wants to do the match, he gets the limelight he deserves. But to do a Flair like exit after his last match, I just can't see him doing it without Vince and Trips pestering him. When you're done, it's time for others to shine.