I am a big Mongol fan, and… I have to say that there is SO much more room for the Mongols to grow.
So, the Mongol religion STRONGLY believes in reincarnation. At the same time, the mangudai are the core of the Mongol army. So… here is an idea.
Instead of the khan simply resurrecting, the Mongolian shamans would need to find the reincarnation of the previous khan. How does this work? When a khan dies, a random mangudai would receive a buff called the reincarnation of the khan. Then, that mangudai must go to a prayer tent to become… the khan. If the mangudai dies before reaching the prayer tent, another mangudai is selected. If no mangudai exists, the first mangudai trained will become selected. This would create a mini empire task surrounding the resurrection of the khan.
Second, once a khan is reincarnated, their units consider them a spirit being. The khan has an aura that gives all members of their army slow passive healing, and they protect the khan with their lives. 90% of the damage that the khan takes is redirected to a friendly unit in range with the highest health.
This is NOT a buff to the Mongols. First, the healing kind of effect already exists. The Mongol’s structure Kurultai already gives this buff. This is just a mobile version of it that is in battle, and that’s not even that much of a buff since the kurultai structure is mobile, and all this buff does is allow a user to get the same result with less micro. It is literally 1 hp per 2 seconds. Its not going to decide a battle. Also, this is a slight nerf to the kurultai. Other Mongol wonders allow early tech research, the kurultai is unique in that it gives this aura. Giving the aura from a second late game source would make the kurultai just like all other Mongol wonders.
Second part of the buff with the khan having more survivability at the cost of nearby units is also not a buff, since the khan does trivial damage, and you should be firing at the damaging mangudai he is riding with anyways.
Third, this makes resurrecting the khan a bit of an exercise, and you trade the micro of packing and unpacking the kurultai, for the macro of finding the khan’s reincarnated successor. Both involve lots of clicking so its not even a buff to the amount of clicking a person has to do to get the same result. This is all a very balanced way to implement something more interesting without messing with any of the power balancing of the Mongols at all.