Have Mosaico dockable to a particular screen-edge, and hotkey-able to display it as well.

It would be a nice feature to have Mosaico dockable to the screen-edge, with maybe 4 or 5 pixels showing it's docked location so that all you would need to bring it up would be to hover at that edge of the screen and within the designated 4 or 5 pixel area and Mosaico would then grab hwind focus and slide-out. Making this kind of tool as unobtrusive as possible is going to be a key feature - making the whole process as transparent as possible.