There is very little that needs to be improved, as BigCommerce is very proactive and is continuously releasing new updates. They are very effective at modernizing their product, but since no product is perfect and can always improve, a few things that I think BigCommerce could do better is to create a better reporting module for merchants.

There are apps available that help with this in their app store, but it would be nice to have this built-in for things such as running reports for sales tax reporting.

The theme editor that was recently added is wonderful, but it not totally intuitive.