The Trinity Adored by All Saints: Detail, the Throne of Mercy

Perhaps Mary's gesture can explain what was intended to be placed on the right side of this image. In some Annunciation images she will hold one hand to her breast in a gesture of humility, so it is possible that the crossed hands in this image are also about humility and that the missing figure on the right was to have been the angel Gabriel, who is normally separated from the Annunciate by some sort of architectural feature.

The Trinity image is of the "Throne of Mercy" type common in the late middle ages and after. The two figures in the interstices of the throne can be identified as angels, although their wings are very faint.

This image in full resolution
View the entire altarpiece

More of the Trinity
More of the Annunciation

Source: Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.