A hypothetical voluntary annexation is equivalent to a messy divorce. Some open-ended questions include
1) What happens to the private property owned by citizens of Ukraine? Citizens of Russia? Citizens of a random third-party country?
2) How is government property transferred? There's some obvious non-commercial real estate here (parks, beaches) as well as commercial (government buildings)? What happens to a property publicly financed by bonds (stadiums, airports, railroads) or other mechanisms?
3) How's national debt handled? Crimea historically attracted more government subsidies than the tax revenues collected, so in a sense it played quite a significant role in formation of that debt. Do certain obligations by Ukrainian government get marked as Crimean, pending the bondholders' approval? Does the working group define some pro rata share of the debt everyone agrees on? Or does Crimea gets to start with a clean slate, while Ukraine gets stuck (ha-ha!) with the bills accummulated over the decades?
Even with an amicable separation some of those issues could take years to figure out (Grexit and Brexit being good examples of complexity). Complete absence of any work in this matter shows that Russia was less interested in establishing its rule of law over Crimea and more in destabilizing its parent entity.