Similar to the already existing Split and Duplicate "policies", QGIS now supports Merge Policies, which can be configured to determine the initial values in the Merge Features dialog.
Policies can be set per field in the Attributes Form of the Layer Properties dialog, under the "Policies" section of the field configuration.
Policy methods include:

• Sum: (Numeric fields only) Sum of values
• Geometry Weighted: (Numeric fields only) Uses weighted average by geometry
• Default Value: Uses the default value set in QGIS
• Unset Field: Clears the field and uses the default value from the data provider (if it exists). If no value is specified, the value from the first feature is used (current default)
• Largest Geometry: Uses the value from the feature with the biggest geometry (determined by length (line types), area (polygon types), number of parts (multipoint types))
• Minimum Value: (Numeric fields only) Lowest value
• Maximum Value: (Numeric fields only) Highest value
• Set to Null: Sets the value of the field for the feature to NULL