What ingredient adds the sweetness on Croissants in Europe?
Sometimes, in the less refined areas, you'll get pastries with sugar syrup brushed on top with the egg. It gives a shine to the pastry that lasts for a long time, protects it from losing moisture and makes it sweeter.

This is why all the industrial pastries are usually so sweet.

I don't think that any traditional European patisseries have ever used Agave for anything, and I hesitate to think what their reaction would be if you suggested it.