Based on surveys that I have seen, probably the right answer would be that Ukraine propose that Luhansk, Donetsk, and Crimea are provided with a free election to decide whether to rejoin Ukraine, separate into Ukrainian and Russian regions, or become independent states (or an independent state).
The voting method will be chosen by and overseen by an organization chosen by or created by the agreement of the governments of Israel, India, and Switzerland. Similarly, this organization can set rules around campaign contributions, political ads by foreign groups, etc. with the power to issue international indictments, global warrants, and prosecute in the International Court. And, above all that, they can void the election results any number of times until they are satisfied that there has been no election interference by any external party.
As a part of voting, a poll tax should be collected sufficient to finance a years worth of time for the following body to produce their work.
The goal is to elect a body of representatives from each region (say, one representative per 10,000 persons) who will be given the vote to answer this question. Along that lines, they will have the power to call for delegates from Ukraine, Russia, or any other body, individual, or organization that they believe could help them make a determination (be it on the basis of finances, defense, or other considerations).
The discreet regions can decide to join together, go their own way, split into subregions, etc. Should they decide to become independent states then they are free to generate and begin their own governments, by whatever rules they wish.
Ultimately, it is the affected people who should, of their own free will (and given the freedom to properly investigate and deliberate), decide.