Isn't Nekusar frequently cited as one of the most hated generals to play against? Maybe less so since some other powerhouses have come along, but I don't think constantly wheeling them and inflicting massive and rapid amounts of damage against which combat can't defend is all that much of a nice experience for new players. Not saying you shouldn't build Nekusar since he fits quite a few of your other criteria, but I don't think being nice to new players is something he does well. I'd rather go with Kynaios and Tiro of Meletis for that.
Things I like to and want to be doing: Stealing/copying opponent's cards, self-mill/graveyard strategy, life gain/life drain subtheme would be neat
In my opinion, The Scarab God would be the perfect commander for you. It's resilient because it's very hard to kill and can use another player's graveyard if yours happens to get exiled. This also makes it versatile and hits your marks of stealing opponent's cards and using a graveyard strategy at the same time. You will often win through whatever win conditions your enemies happen to bring along, ensuring each game is different. It doesn't gain life, but it sure as hell can drain your opponents for a lot. And while it's often played as zombie tribal, you don't need to go down that route. You can also use amazing value creatures like Sepulchral Primordial, Diluvian Primordial and Sphinx of Uthuun. Then maybe pack a few really good zombie generators like Army of the Damned or Grave Titan to drain your opponents out. Feel free to have a look at my decklist if that sounds fun to you!
In my opinion, The Scarab God would be the perfect commander for you. It's resilient because it's very hard to kill and can use another player's graveyard if yours happens to get exiled. This also makes it versatile and hits your marks of stealing opponent's cards and using a graveyard strategy at the same time. You will often win through whatever win conditions your enemies happen to bring along, ensuring each game is different. It doesn't gain life, but it sure as hell can drain your opponents for a lot. And while it's often played as zombie tribal, you don't need to go down that route. You can also use amazing value creatures like Sepulchral Primordial, Diluvian Primordial and Sphinx of Uthuun. Then maybe pack a few really good zombie generators like Army of the Damned or Grave Titan to drain your opponents out. Feel free to have a look at my decklist if that sounds fun to you!