One of my favorite movies is Batman Beyond Return of the Joker and my favorite twist is Tim being the Joker part of why I like this part of the story is how dark it is and I've often wanted a story line like that to be incorporated into the comics. I feel a good twist on it would be for Tim to be infected with a seemingly "incurable" new version of Joker venom and for it to drive him insane. Like Joker seemingly meets his end and in his final moments he hints he has one more scheme then he dies and after brief moment of peace Tim gets a package and opens it thinking it's not dangerous and then gets sprayed and inside is a message from Joker saying "I thought about giving this to Nightwing then I thought Jason might be more appropriate then I realized it would be so much more fun if I gave it to you so you could be my ultimate revenge."

I feel like Tim would make a good villain because he's a genius and just imagine if he turned against the Bat Family besides if Joker really wanted to hurt Batman what better way to do that than corrupt the Robin that succeeded Jason? Also an easy retcon would be a speedy clone type twist that way both Junior could stay around and Tim could be absolved of his crimes. I'm assuming DC would pull a retcon like that especially if Tim became Junior and did some really depraved things. As for the personality I think if Tim became Jr. it would be interesting if he didn't become completely like Joker but more like Jeremiah from Gotham with a serious yet unhinged personality.

I think Tim gone bad would be interesting to see and it would be interesting to see Batman in a situation where one of his Robins for the time being is beyond his help and he's forced to watch them become a monster.