He could be better, but I don't see anything to suggest he could extract any more turn than he does. If he were to make impovements to his game, it'd be in his use of loop and drift - this'd hopefully mean he could tie knots around good as well as moderate to poor players of spin on turning pitches (and yes Jack, MSP can turn the ball on pitches that some fingerspinners wouldn't, but there's still a great deal that he can't turn it on much) but it'd not make him any better on the sorts of non-turners he's been ineffective on.
Nah, even Harbhajan and Saqlain needed a turning pitch to be effective. It was just, at their best, they were almost impossible to stop from getting a stack of wickets if you got on a turner. Because not only did they turn their Off-Break big and have an alternative delivery, but they were skillful flight bowlers as well. Both have in recent times lost some of that flight skill, and it's impacted upon their effectiveness in Tests (and Saqlain indeed has lost more than that and these days struggles in English domestic FC and OD cricket where once he ripped to shreds).
Apart from Saqlain and Harbhajan, there's no-one who's capable of bowling the Doosra who's all that good. There's a few low-level bowlers (Daniel Cullen, Shaftab Khalid, Johan Botha) who've reputedly developed it to some extent, but none are Test-class bowlers or close to. Not even sure Shaftab is still in the game, haven't heard of him for a good few years.