Example:
Let's say a German supermarket sells a chocolate for cash 1.00 Euro (net), 1.16 (gross). The VAT (value added tax is 0.16 Euro (16%). (FYI: In Germany when an VAT taxable service or good is sold the merchant who sold the product/service has to inculde the VAT as shown in the above example. At the end of the month he has to transfer the VAT to the fiscal authorities / collection office. I assume that this VAT system is used in many countries. So now back to the example:). 
The supermarket accountant enters a new split transaction in GnuCash:

Bank(+) 1.16 -> Sales[VAT16 %] (+) 1.00
                    -> VAT(+)     1.16

The problem with this is that the accountant allways has to enter a split transaction when an VAT taxable item is sold. That can be a lot of work! Therefore I would like Gnucash to automatically create a split transaction when a VAT taxable product/ service is sold allowing the GnuCash user to save a LOT OF TIME if he has many transactions (like a supermarket usually does). So when the accountant enters a new account like "Sales" GnuCash should allow him to define whether the sales are VAT taxable and if so what is the VAT percantage. Then each time he books to this account the transaction is split into sales and a separate VAT account.

Adding this feature to GnuCash would probably significantly save many accountants a lot of time!