Thanks for the great work on the multires modifier 
I have few suggestions :
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I wish sculpting could modify the base mesh/level one of the mesh ; for the moment, whatever I do to the level 2, it doesn’t change it. I guess the goal is to leave the base mesh intact, so what about a level 1.5 between 1 and 2 that would be the 1 modified accordingly to the upper one, and an option “apply only 1.5” to apply this to the base mesh?
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the edit mode is not available anymore for the other level that the level one
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in the version of blender that doesn’t integrate your work, I use the option “display wire” (object->draw extra->wire) to see the base topology of my mesh. I miss this option “edges:1,2…” to choose on which level I want to display the base wire.
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“save in old fashion way” : when I open an old file that had multires (in a not modifier way), it creates automatically a modifier with those information. I like this, but once my file has been edited with the “multires as a modifier” version, I can not have those informations in my “old fashion multires” version of blender
I hope my feedback is useful