So there's little chance speechcraft will be in ESO but:

1). Would you have enjoyed seeing it in ESO? ...with Players or limited to NPCs?

2). How would you have added it in ESO?

I always thought it would be neat with a free for all system BUT you couldn't outright attack someone without 'starting an argument' first - (unless the character has no speechcraft skill at all, then it would be 'attack on sight'). It would replace a PvP flagging system but also double as a fighting mechanic.

Illusion magic could tie into speechcraft so if you have low personality then you would be more subjectable to calm effects (preventing you from attacking for a short time), charm/mind control (forcing you to attack someone or harm yourself) or other illusion abilities; Demoralize, Frenzy, Rally or Silence. These abilities could be limited to convesation only as not to upset any other PvP systems.

Don't get me wrong, if added incorrectly these could be very annoying features but we're just having fun here.

Vanguard had an awesome speechcraft game which they call diplomacy. More than a mini-game, it was an entire sphere almost equal to the combat sphere. The only thing they missed was allowing player vs player diplo.

What about the barter part, how would it affect trading between players ?
Player A(Speechcraft level 10) setting shop : sells Teeth of Ogrim 100 000 gold pieces.
Player B(Speechcraft level 100) buys Teeth of Ogrim from Player A, but only pays 66 000 gold thanks to speechcraft-barter resolution.
Player A:" What The Fishy stick!"

Yes.