Remove all the objectively (from the perspective of a non-oblivious user) stupid stuff that 10 and 11 added including half-baked UIs, extra clicks, inconsistencies, random forced changes, 'AI', etc.
How many of us have encountered 'Legacy' systems that get a bad wrap but are actually responsible for everything?
Eventually Legacy will have a positive connotation. So let's get to that now and resume improving the internals unless there's actually something else to do.
And for those who bring up being afraid of change I propose Windows Random Change edition for you.