Client commands like Remote Commands

Ability to execute client commands on local computer e.g.
clientprogram.exe %1
clientprogram.exe %1 %2

where %1 and %2 are selected files. If either file is on a remote server then download the file to temp location before executing command.

Alternatively add scripted command "&exec" and tokens "%l" to the current remote commands to achieve the same thing. e.g.
{
GET %f %f.tmp // get file as
&exec "clientprogram.exe" "%l" "%f.tmp"
}

what's a specific use-case for this?

You upload a text file on Friday
Someone changes remote file on Saturday
You attempt to modify and upload your local copy on Sunday

At this point the datestamp and size would be different, but overwriting the file would cause Saturdays changes to be lost.

Running a client command on the files would allow you to see and merge changes.

It sounds like all 3 of your recent suggestions are looking to solve a problem that should be handled by source code version control systems such as cvs, svn, github or a million other projects.
If you use one of those systems, you will not have problems of overwriting unknown changes.
Trying to solve this problem using an FTP client is a wrong approach.

I know what you mean, but it's not to solve that. It's solve the problem that with flashfxp we cannot run our own client commands on local and remote files, a simple feature.

It could be for:
A: Diff/merge
B: Accounting Changes
C: Applying business logic to updated folder data
D: It doesn't matter basic fact is, anything outside the realms of FTP cannot be achieved because flashfxp does not support a simple feature allowing client commands, whereas many other FTP clients do.

It's not exactly difficult is it. and it'd make flashfxp a lot more powerful.