A few things I would really like to see in Multiplicity..
Sync screensaver states between all machines and between machines in use (primary and whatever secondary is being used)
Ability to detect whether keyboard and mouse are present and automatically switch the machine that has them to the primary. I have more systems than I do monitors, so some are shared by a KVM switch.. it would be great to be able to have MP detect which machine I'm using on the KVM and make it primary, and do so seamlessly.
I know this is probably a huge request, but: application sharing. I've been trying to think of ways to combine my networked computers into a more cohesive group and recently saw an application server that allows a client machine to run a program from the server machine, showing the UI on the client (via Terminal Services) while the server does the actual processing, and thought this would be perfect for Multiplicity. I would imagine that you could use this so that you could drag an open window between machines. Like I say, I'm sure that's a big request, but it would make Multiplicity something of a "holy grail" for me, practically erasing the line between two individual machines, and greatly enhancing the dual-display style setup.
A clipboard manager so you can view and choose what to transfer between machines. It would also be really nice if you could have multiple items in the clipboard to choose from. That way you could copy different files from different locations to the clipboard and copy them to another machine all at once. You could also copy a hand full of files and just different ones to different secondary machines. Additionally it would be very nice if you could put shortcuts to other ThinkDesk components in there, so you could highlight a file and then have KeepSafe or ThinkSync take care of it after copying. Drag and drop functionality would be very welcome here, as well as the ability to seemingly drag it between machines. So for example you would open the manager, drop some files in it and copy some text, then just drag the box to the other machine (it wouldn't need to be perfectly smooth) then drag the files you want out into the desired location, at which point it would do the actual copying.
Interface with Theme Manager to keep synchronized themes between machines. For example make it so you could load a WB skin on a primary and secondary system at the same time, and if you select a wallpaper it could split it and set each half to the respective screen.