For Custom Commands, an option to "Show Results as message" would be useful on pass/fail commands with simple but significant output; for the message box, set the Title to the Custom Command's Description, the Prompt (or Content) to the command's output, and show an "OK" button. I can envision commands that need something done in the Terminal if they fail, so also having a button to dismiss the message then open the Terminal window could be useful.
Rationale: For many simply commands I neither need nor want the overhead and delay of opening the full Terminal window, which is only going to be immediately closed.
One such command is my "Check PHP Syntax":
It would also be good to include in https://winscp.net/eng/docs/ui_customcommand
a tip that appending
to a remote command will suppress any non-zero Return Code from the command, which allows the output to be shown in the terminal without going through the "Error executing custom command ... Abort / Retry / Skip / Skip all / Help" dialog. This tip works on Custom Commands in existing versions of WinSCP, but perhaps a future release can give Custom Commands an option "Ignore Return Code" which would simply bypass this dialog.