Crimea held a referendum for independence from Ukraine in 2014. It passed by 95%.
If anyone has any doubts about how the referendum would turn out in the Donbas, they only need to look at this to have their questions answered.
And Ukraine gets the right to join NATO. That's a face-saving result for both sides that has real potential as a permanent solution.
I'd point out that Ukraine already, as a matter of policy, is on record as supporting elections in the Donbas under the Steinmeier Formula. So it is a matter of ironing out the details, and perhaps some compromise regarding the processes could be reached.