We already have the capability to become an undead summoner with Frostmourne, Leoric's Folly, and Diablo's Rebirth. Why not just pull the trigger and make an undead Pally a class all on its own? You could do away with half of the Pally's auras and combat skills without anyone noticing. Just merge the remaining functional auras onto one tree and then add a summoning/poison tree.