Immortal Bankers

My suggestion is to make the non-accessible bankers in cities such as Stormwind or Silvermoon immortal & deal an absurd (deadly) amount of damage when hit. The only way (to my knowledge) to get to the bankers in Stormwind is via exploiting the map of the game & adds nothing but annoyance when a person does it. Bonus point if they are a stealther and cannot even be caught for long enough to be SS'd and reported.

Then what's the incentive for players to kill the invaders?

You can't kill the invader unless you make use of the exploit yourself, which is essentially saying that "two wrongs make a right". Do note, I said BANKERS, not auction house vendors. Also, specific bankers - not the ones that are openly available, like the ones in Ironforge, for an instance. I mean ones you cannot actively defend without breaking the game's rules yourself.

A better suggestion would be to fix that exploit.

The exploit to fall under the map and walk to the banker from there, would otherwise be useless, if the banker one-shot instant-killed them. Furthermore, the former exploit would take a lot more (needless) effort to fix, as is an issue with the overall making of the map. Take away their REASON to exploit and they will stop exploiting. Of course, if you can, stop the exploit, but considering one is a so much easier fix than the other, I simply don't see the reason to go through the hassle.

if they can glitch their way under the map they can surely do more than just "kill a banker". The exploit problem seems far more concerning than making a few NPC inmortal because a player (or several, idc) cant walk to the nearest city or deal with the threat.

lets assume for a second that your idea gets approved: fine! the bankers are now inmortal!. Oh wait, the ones that glitched the map are attacking some other NPC, like the SW king or the sort. you're gonna make him inmortal? then they attack the players! are you going to disable pvp in the cities?.

Just go to the root of the problem and fix the exploit.