Slanting and asymmetry after braces. Is there anything that can be done about this? (Photo)

I see what you are talking about, your orthodontist should as well.
It is your right to insist the braces stay on until you are happy, you contracted for a result, treatment time is only an estimate. If it takes longer than expected to get said result, there should be no extra charge.
The appearance of the "slant" is actually caused because your two front teeth are slightly different shape. the left side tooth (your left side) is more tapered than the right.
To fix this, the right side tooth should be "narrowed" a bit and it might actually be necessary to do some cosmetic bonding to the side of the left tooth to make it look exactly the same as the right one. Work on the right tooth first and see how it looks, then if bonding is necessary the orthodontist may need to open a small space to allow the dentist to do his work then close it after. Making the two upper front teeth perfect is the absolutely most important aspect of your smile. Do not settle for anything less that 100% satisfaction. It is impossible to fix later with out going back into treatment. Your orthodontist may grumble but don't be put off. It will be uncomfortable if you feel like you are having to stand your ground but you will be happy you did.