If I were Vladimir Putin, here is the end game strategy I would pursue.
Russia and Putin want the Crimea because it is the location of Russia's only warm water naval base. The Crimea was part of Russia starting in 1783 and only became part of the Ukraine in 1954 when the then Communist dictatorial rulers in Moscow, for administrative reasons, transferred the Crimea from Russia to Ukraine.
The Ukraine is dependent on Russia. Its economy cannot survive without Russian voluntary cooperation with Ukraine.
The Ukraine gets almost all of its natural gas energy from Russia. Russia subsidizes the cost to Ukraine of the Russian gas Russia supplies to Ukraine. Russia could double or triple the price of the natural gas it sells to Ukraine and destroy the Ukraine economy in a matter of weeks or months. Alternatively, Russia could cut off supplies of Russian gas to Ukraine, freezing Ukrainers in their homes and offices within a matter of days or weeks and destroying the Ukraine economy. Does anyone think the EU and USA taxpayers would want to send tens of billions of dollars of additional economic aid to the Ukraine so the Ukraine could send those tens of billions of Western taxpayer dollars to Russia to pay for Russian natural gas? I don't.
Russia knows the weak Ukraine military is no match for the occupying Russian forces. No one in the EU or USA will support military action to oust the occupying Russia forces.
The USA very much needs Putin's cooperation on three matters of greater importance to the strategic interests of the USA than whether the Ukraine or Russia control the Crimean peninsula: (1) the West and UN cannot complete the Syrian disposal of Syrian chemical weapons without the active cooperation of Russia; (2) economic sanctions against Iran - the "stick" that is currently motivating the new Iranian government to (continue below the orange thingy)
engage in negotiations to end Iran's development of atomic or nuclear weapons - will end in a few months without Russia's cooperation, because Russia's can use its veto in the UN Security Council for any proposed extension of the economic sanctions; and (3) Russia has some limited influence over the murderous, uncivilized North Korean regime. Russia is not nearly as important as China to the North Korean leaders, but North Korean leaders travel by train to Moscow every few years to seek Russia's limited support of North Korea.
But Putin still needs a fig leaf of legitimacy for its military invasion and occupation of the Crimea.
He could call an election - a free and fair election, one transparent and overseen by UN or other western election monitors. The voters of the Crimea would be given a simple choice by secret ballot to either become an "independent" republic within the Russian Federation or remain a part of the Ukraine. Russia would announce in advance that it will honor the results of the election. There is no risk that Russia and Putin would lose such a free and fair election because the voters in the Crimea are overwhelmingly ethnic Russian (not Ukrainians) and in recent Presidential elections in the Ukraine the Crimean voters have overwhelmingly supported the pro-Moscow candidates and not the pro-European / pro-West candidates.
Recall that after the fall of Communism in Czechoslovakia, that country had an election to decide whether it should remain as one nation or divide into two separate nations - the Czech Republic and Slovakia. They voted in a free, fair and transparent election to divide and no one in the West challenges the result.
After Crimea voters vote to become part of Russia in a free election (probably overwhelmingly), Russia could announce that it will abide by the results of the election. The Crimea could announce that any of its citizens who wish to immigrate from the Crimea to the Ukraine are free to do so. Russia and the Crimea might even agree to compensate the Ukraine for any Ukraine property in the newly Russian Crimea. [It would be far cheaper for Russia to bankroll this than to continue to subsidize the Ukraine's purchases of Russian natural gas and/or to continue making tens of billions of dollars (equivalent) of Russian government loans to the Ukraine to prop up the Ukraine's basket-case economy.]
The European Union, the USA and the Ukraine would not like that outcome but they have almost no leverage over Russia and Putin if he elects to continue his military occupation of the Crimean peninsula (Crimea only, not any territory of the Ukraine beyond the Crimea). If there is a free election, the EU and the USA will have even weaker arguments against the status quo.
The USA and EU may cancel some meetings and apply some limited economic sanctions against Russia for months or maybe even a few years, but the Crimea's annexation by Russia will probably last at least as long as, for example, Israel's annexation of East Jerusalem (starting 1980 and continuing).