If Karan Johar made Taare
Obvious starcast:
Shah Rukh Khan as the arts teacher
Aryan Khan as the dyslexic child (even if he could not act for nuts).
Rani Mukerjee as the kid's mom (assuming Kajol is unavailable).
Abhishek Bachchan as the kid's dad.
Amitabh Bachchan as the school principal (who cares if the role is ultra minute, he can afford it).
It would be shot in New York to appeal to the NRI audience.
The story line would obviously be different. SRK would fall for the dyslexic kid's mom. The last scene would have the mom running to the teacher rather than the kid. And again, like in so many other movies, SRK would get someone else's girl.
It would have one dance number.
The film would be titled 'Kuch Taare Zameen Par .'
If Sanjay Leela Bhansali made Taare

Obvious starcast:
Salman as the teacher.
Rani as the mother.
Of course the whole film would be shot on elaborate sets. The school would be nothing short of Harvard university.
An orchestra would play every time anyone cried.
Slow motion, different camera angles for every scene.
The school uniforms would match the classroom walls even though that does not make a ny difference.
The film would cost 60 crores.
If Farah Khan made Taare

Obvious starcast:
SRK as the teacher (yawn).
In the original Taare, Aamir makes an entry at the interval point. In Farah's version, SRK would be on screen on for 2.30 hrs out of the 2.45 hrs and would be introduced in the first scene itself.
The story would be changed to make sure the above happened. The focus of the film would be a teacher who helps a kid fight dyslexia.
To make it a complete entertainer, there would be a romantic angle, comedy, and action thrown in. Oh idea!! Nikumbh's character likes another teacher and the kiddo helps him.. throw in some comedy moments there and you have romance and comedy settled. For action.. hmm.. lemme see.. oh yah, the kid gets kidnapped and the teacher fights the baddies to save him. Wow!! I'm quite an imaginative writer. I can see how Farah can write a film from scratch in two weeks straight.
The film posters would have a big SRK with the tiny image of the kid in the background.
If Rakesh Roshan made Taare

Obvious starcast:
Hrithik Roshan as the teacher.
Since Rakesh Roshan cannot think beyond science fiction these days, this film would have that too. Instead of dyslexia, the kid would have alienositis or something, a condition induced due to him witnessing an alien abduction.
Instead of Nikumbh being an arts teacher, he would be a physics teacher, and instead of asking kids to be creative, he would ask them to challenge the science we know.
In the scene where Nikumbh asks the kids to open their minds and make whatever they want outdoors, the kid Ishaan, instead of making a boat, would end up making a working spaceship prototype.
Nikumbh would cure the kids problem by making a full fledged version of the kid's prototype, traveling to the alien planet, and asking them to give the kid his powers back.
The film would have music by Rajesh Roshan ripped off from some world music.
The film's name would again start with a K.. probably ' Kuch Aliens Taaron se Zameen Par' .
The director would make sure Hrithik gets to show all his abilities. This would mean a scene with Roshan jr flexing his muscles, and a dance competition in the end, instead of an arts competition.
If Priyadarshan made Taare:

Obvious starcast:
Akshay Kumar as the teacher.
Paresh Rawal as the kid's dad.
It would be a brainless comedy. The kid's dyslexia would be made fun of. Half the times the parents will be running after the kid from one room to the other and that, in the director's opinion, would be funny.
In the climax of the film all the characters in the film would run around in the amphitheater for no reason, spilling colors on each other. That's where the film will end, without any logical conclusion.
And of course, Paresh Rawal would emote like an epileptic himself making us question the boy's mental abilities anyway.