I don't see any [problems with the concept of "territories which don't want to be Ukrainian"], but you are welcome to point out some if you think you know them.
I think it is best, most educational for you to try to figure this out yourself, with some guidance.
Now, in our modern age, it seems logical (to me, at least) to make, from time to time, some border adjustments to take the sociological reality of said territories into account (one may think about Kosovo, Crimea and so on).
...
Ok, please take out a blank paper with the pre-2014 borders of Ukraine drawn on it, and then mark with a red pencil the "territories which don't want to be Ukrainian", taking into account "the sociological reality of said territories".
Or rather, explain in some actionable detail how exactly you would go about identifying those territories so you can put them on a map.
You may want to consider how "the sociological reality" has been
changing - since 1990, since 2000, since 2014, since 24 Feb 2022 - and how those changes should affect the territories you choose to mark red.
You may want to think about what
units of territory to poll - oblasts? Cities? Individual villages? Individual residential plots?
You may want to think about how and when to poll territories.
You may want to take into account that the "very bloody battles" you wish to avoid have already been forced upon Ukraine by Russia - and whether you propose that, as a general rule, the sociological results of such bloody ought to be heeded - that indeed, according to you, bloody battle is a legitimate and advisable way to change sociological realities and borders. Or why else would you place the burden on Ukraine, not Russia?
Go!
If you cannot, without further ado, describe in your very next post that process and consequently suggest a definitive map, then I propose you
do have problems with the concept, and merely feign not to see any.