There are times where I want to tag a different set of files but don't want to clutter up the current instance and don't want to close the files I already have loaded. There are also circumstances where I am dealing with a very large set of files, and an unexpected crash or power flicker causes the program to close abruptly, requiring me to match hundreds of clusters to releases all over again. For these reasons, I would find it very advantageous to be able to save the current Picard session or project to be reloaded later. 

In my vision, a "session" represents all of the currently loaded files and tags, which would include any uncommitted/unsaved tag alterations. Loading a saved session would not affect scripts or plugins, but it would affect the state of the "Rename Files", "Move Files", and "Save Tags" checkbox items under the Options menu. Ideally there would be settings for automatic session back-ups and maybe loading the last opened session on launch added to the Options window.

The new menu items might look something like this:

File > Load Session...
File > Save Session...
... at the bottom of the menu, above Exit ...
Close Session

"Load Session" lets you select a MusicBrainz Picard session file (*.mbps perhaps).
"Save Session" lets you save a MusicBrainz Picard session file.
"Close Session" closes the current session. If there are files open, Picard asks you if you want to save the session before closing.

I had a look but I'm not sure if there's a ticket for something along these lines already. Maybe there are serious flaws with this idea I'm just missing. Either way, I hope this all makes sense.