Lilya Horowitz, DDS

Hello!

Based on the description of your teeth, it sounds like you might not be able to achieve your smile goals just by lengthening the two front teeth. This CAN be done, but there is a high risk the restoration will fail, whether it is bonding or veneers. When we increase the length of teeth, we are changing your chewing pattern and if we don't make the proper adjustments to your entire mouth, you will end up breaking the teeth that we made longer because you won't be used to them. 

This can be corrected very easily with dental bonding! when done in the right hands it will be look natural and last for many years and best of all, not extra tooth structure will be removed!

I would not be too concerned about the longevity of the crown but as far as appearances the dark line will not go away. Even though Zirconia is "white" not metal, the dark line at the gum can be caused by any thick material. What happens is the margin of the crown is not translucent (metal or very opaque like zirconia) and it blocks the light, causing a shadow, which gives the appearance of a black line at the gumline. The only way to eliminate the line is to have the crown remade. I hope this helps!

Bonding can be very successful
but if the gap is large your two front teeth may appear to be very large after the bonding is completed. I would advise having a mock up done first so you can get a preview of the final results. 

You can achieve this look with bonding quite easily, and if you are ok with the proportions then go for it but it will make your two front teeth very large.