Actually you can achieve this using synchronize function in mirror mode.
Just that you do not have the rename option there.

Actually, a TABLE would be a great idea. On each line of the table, it could have a drop-down list box with actions for each conflicting file, and a textbox for a new file name (if the rename action is desired).

However, I know this might be a lot of trouble, especially because it is not a standard GUI control.

All that is REALLY necessary is for the program to display the same dialog box that it does now when a conflict is found. What I am saying is that it should show this dialog box repeatedly (once for each conflict), BEFORE any transfers are actually started.

(By 'conflict' I mean we are uploading/downloading and the remote/local file already exists.)

So you want WinSCP to looks for all files to be transfered and display e.g. table with all conflicts?

When one initiates a transfer of multiple files, the user should be asked what action to perform for each file in the transfer if it exists locally.

This should be done before any of the transfers occur. Otherwise, the user has to sit and wait for each of the transfers to complete before being asked what to do with the next file.

If the user is asked about each file (individually) before the transfers begin, then the user can work on other things while the transfers are processed.

Of course, one could click on one of the "For All" buttons, but what if the user wants to perform different actions on the different files?

WinSCP should know in advance if a local file exists that would be overwritten by a successful transfer, and so it should be no problem to ask the user what he wants to do in each case.