Quick note, man. I work 40 hrs a week in a customer-service job and I have customers telling me what I "will" do for them all the time. When you write "when you have done this" it implies that I will implement your ideas, and you're turning my hobby into my day job which just ain't cool. If it wasn't what you intended, then please just be aware of how it was received.
As far as what you requested, I like your ideas!
1st, making MAST compatible with Unity 2018 LTS is a no-brainer. Thank you for that information. I'm still a newb at most of this. I'll get it added and change the Unity version scope in the Asset Store in the next release.
2nd, I'm already planning on changing the way the prefab palette works. Right now, "Load from Folder" grabs all prefabs from any selected folder, including subfolders, and places them all in a single array that the palette uses. This will be changed soon to create a list of palette arrays that can be changed with a drop-down at the top of the palette. The dropdown will be a list of "folder/subfolder" names. Still, before I implement this change, I want to redesign the palette storage so they truly stay persistant. I sort've addressed this in your other post concerning the problem you've already witnessed yourself.
I mention all of this because you spoke about grouping by types and actually keeping the prefabs in separated folders would take care of some of those needs I believe.
In addition to what I'm planning, I could also add an integer to the MAST Component Script for "groupID". This would be a user-defined value that would always default to 0. I could then add a hotkey to iterate through a list of items that all have the current item's GroupID.
The idea of randomly swapping out an item with another sharing the same GroupID makes sense. If you're in placement mode, it will swap the item being placed and the ghost visualizer will immediately change to a new random item from the same group. Otherwise, if not in placement mode, it will perform the action on whatever item is selected in the scene and register an undo.
I like these ideas and I think they will make MAST more useful without making it too complicated "if implemented correctly". I just don't know how far down the pipeline it will be.
Lastly, I haven't attempted to capture a mouse scroll wheel event, but I love the idea of using SHIFT + SCROLL WHEEL to iterate through each palette item for speed and convenience. I will try to implement this in the next release.