I guess this is not a matter of negotiations. Putin's aim is to concur Ukraine. The question is if this will inclusive the western part or not. So nothing neutral or plebisciet that were options of the past, not in the current war.
Its pretty clear that there is a general problem between the Putin and Selenski regime. They are at odds with each other and Putin want a new Ukrainian government. If this would come, he would agree to the mentioned terms I guess. Problem is, that Selenski was very smart in positioning himself nationally and internationally as the defender of Ukrainian sovereignty, so his position is now better and more entrenched than it was before. Since his regime doesn't really care and will probably go on to the bitter end, we are facing a very problematic situation in which both sides are pretty much all-in and are not really open to serious negotiations. Especially Selenski probably never was, which caused all that mess together with Putins overreaction to the constant provocations. That's a dire situation.