As the title suggests, it'd be nice if we could merge more than two surfaces at once.
The current way to merge 3 or more surfaces is by creating multiple merged iterations, each with their own name, and their own set of button-clicks.
A concept for this could be taken from the existing Join Lines command, which includes the option of joining only Two Lines (Merge Two Surfaces), or joining All Selected Lines (Merge 3+ Surfaces).
There's usually a base surface that all additional merged surfaces should overwrite, so a variation of the "Finish replaces existing" merge type could remain. Or, a new dialogue box could open which would allow the user to add surfaces, and reorder them by level of priority; i.e. the surfaces at the top of the list are overwritten by the surfaces below them in the list.
This would be useful for updating an FG model based on design changes which only affect a portion of the site (sometimes the designer will deliver only surface "patches" rather than updated full site surfaces).
This would also be useful to merging multiple utility trench surfaces together at once.