Involving God of War: First of all, for reference, Hunter's Mercy was officially nerfed in the spell compendium, granting a critical hit on one attack if you hit but not granting a natural 20.
To my knowledge, the only abilities that still grant natural 20s on attack rolls are a single luck feat (which is situational and usable 1/day) and a the aforementioned Surge of Fortune, a spell only on the cleric list. To my knowledge, this means that the spell can only be accessed by Clerics, Archivists, Rainbow Servants, Ur-Mages, and Favored Souls (Tier 1, Tier 1, Tier 1, Tier 1, and Tier 2 respectively). In addition, up until now, you have seemed pretty adamant about providing solid tier 3 material and this seems to be running in strict opposition to that aim.
While it is a valid point that terrible things can happen when a wizard gets the jump on you (such as your time stop example), that example you gave hardly kills players immediately and gives allies a chance to dispel it (and you should note that it kind of uses 3 level-9 spells, meaning that it could probably be done 1/encounter at most in an average adventuring day). Even with the ubercharger, there is at least a 5% chance that the attack will miss and you could survive.
With this feat, an NPC can (at least in some situations) approach a player with a vorpal weapon and say "you die" without any real chance of failure. Apart from not typically being possible (even via spells), this can be positively disastrous if the party doesn't know that standing next to the enemy is a death sentence. Even worse, you give the players the ability to approach an enemy (such as a BBEG who has been established as being humanoid before the player took this feat) and do the exact same thing. While a DM certainly could (and indeed would be expected to) change the circumstances of the encounter to account for this, you have still introduced a disruptive factor that requires the DM to change their entire plans like a Wizard's ability to use Contact other Plane to solve a mystery or to simply cast planeshift and abandon your set campaign with the party to explore Gehenna (well, maybe not that bad). In other words, this is more-or-less the type of disruption that has been knonw as a hallmark of strictly tier 1-2 games.
Even if you give everyone miss chances and immediate actions that let's a player say "I don't get hit", that isn't necessarily fixing anything. Instead, you are simply bringing "rocket-tag" to the masses, encouraging players to stack on immunities and acquire absolute defenses as the first thing to hit them is quite likely to kill them as well.
What was your intention with this feat?