It seems like this is already on the Gnucash developers' radar, but I thought I would add it here as well because it could be a very useful feature.

In the United States, we have to report capital gains on individual tax returns every year --- the gain is reported (and tax paid) when the asset is sold. The tax return includes the item, quantity, purchase price, and sales revenue. Since Gnucash is already tracking lots of identical items (e.g., stocks), it should be possible to gather enough information to automatically generate the information for this tax return -- which can be quite a chore if done by hand, especially for people engaged in active trading.

Another related and useful feature is manual lot identification: If I buy some stocks on January 1, then buy some more on May 1, then sell a few on June 1, I should be able to specify whether the June sale is of the shares purchased in May or those purchased in January (or some of each). This matters for tax purposes.