I would LOVE to see the Undertaker feud against Kane again, only if Kane were to win for once...for a proper send - off to retirement for Kane!!
On a side note, now that it's knocking on the door, is there ANY way to send the Undertaker off to retirement properly?? I mean, he's the DEAD guy, he is immortal (logically), he has been buried, he has been burnt in a casket and yet time and time again, he comes back !! Apparently, this guy resides in DEATH VALLEY! So, him having a Shawn Micheals - Ric Flairesque retirement angle would be breaking of key-fabe! There has to be one fancy angle.
IMO, the best angle to send-off both brothers without breaking keyfabe would be to have their rivalry grow to a ferocious level! Have them beat the hell out of each other, meet up, beat the hell out of everybody, then again turn on each other, win tag titles, compete over the world title as tag champs. Bring back elements of their old storylines...but keep it minimum(don't bring back Paul Bearer coz he's been buried in cement by the Undertaker)! Hell, have Kane kidnap Michelle Mccool and threten to set her on fire! Basically, all this would play on for the next 3 months, to pay tribute to the brothers' legendary carriers, and to set up the following masterangle!
At Survivor Series, Kane and the Undertaker headline in some sort of match (MAYBE a retirement match, with the announcers expressing doubts over the practical meaning of that stipulation). Have Kane lose, but not without several screwups where it would be clear that Kane would win, but some miracle occured and Undertaker survived. Then, have Kane bring up the urn! Have Kane do sth to show that Kane controls the Undertaker's spirit by the power of the urn! Then have Kane beat the **it out of the UT, lastly, have him burn the urn, and BOOM, all that is left of the Undertaker would be ashes and that's the last we see of him. Then, have Kane say sth to the effect of.....'HAHA HA HA HA...13 YEARS LATER, VENGEANCE IS FINALLY MINE! BY BURNING MY SOUL, UNDERTAKER HAD STOOD THE LORD OF DARKNESS! NOW AFTER BURNING MY BIG BROTHER'S SOUL, MY SOUL CAN FINALLY REST - IN - PEACE' Then, have Kane burn himself!!