At this point, I just wouldn't. There is zero interest - zero - for him having anything to do with anyone else. Nothing else would be important, nothing else would be significant.
If I had to, I'd make him GM of NXT somehow.
Exactly. The whole point was to have him go against the streak. it would have been the ultimate match up. Now there is kinda nothing for him to do. If they still booked an Undertaker match there would really be no excitement. It would just be an old man match.