I don't know if this was already mentioned, but it would be very useful to have the ability to put/lock-in more than one plugin per Multi-Dock window. Therefore, one could organize "pages" of plugins based on their usage. For example, if multiple compressors, delays, or reverbs are used in a project, then each variety can be organized into their respective "pages" in the Multi-Dock, perhaps even having the ability to name the "tab" at the top.
The Multi-Dock "page" wouldn't need to snap-to-grid or anything like that. Rather it could be more of a "sandbox" type approach so wherever the plugins are placed, they would remain.