Balance in all things: a design should be minimalist where that is called for (the background, unimportant or repetitive elements, etc.) and should increase in visual complexity/depth/color/brightness in areas that are important and need to draw the users eye. (NB: minimalism does not mean "nothing". Sometimes a slight texture completely changes the feel of a UI, but it requires a deft hand.)

I'm glad to see the flat-ui/material-design over-application of minimalism is coming to an end and designers are getting back to more complex visual treatments. My hope is that, rather than casting back and forth between the various schools of thought, designers start thinking in context-sensitive terms: not "Should I use flat design or skeumorphic design for this app?" but rather "Where in this UI are flat design AND skeumorphic designs best utilized?"