You don't change any of the context that led Russia to invade in the first place. You leave intact an aggressor who has repeatedly and consistently expanded. You gave them Crimea and hoped that would appease them. You tolerated their aggressions in eastern Ukraine. Now you will just give them Donbas and Luhansk and hope once again to appease them?
That strategy might be better accepted if the world hadn't seen 1938-1939.
This tactic worked well for 8 years in the case of Ukraine and was heavily (really heavily) promoted by François Hollande and Angela Merkel.
IMHO, it's good to stick to a tactic with proven track record. The main idea is to feed the monster with virgins. Give them Crimea, then Donbas, then Belarus, then South Ukraine, then Eastern Ukraine, then Central Ukraine, then Western Ukraine, then Georgia, then Finland, etc., etc., etc. With a few years between each scarification, this can keep the monster satiated for a long time, so it will be a problem of your children, while you keep a comfortable style of life. For bang per $, this is the cheapest and safest option for most Europeans and Americans.
There is no "and then" with NATO member sovereign territory.
Ukraine is only being invaded by Russia because it isn't a NATO member.
Russia was willing to invade Ukraine, for some land. Russia isn't willing to invade 28 European countries + US + Canada, for some land.
Where did you even get this idea? Russia is the largest country in the entire world, encompassing 1/8th of the inhabitable land on Earth ALONE. Why would they get in a fight with the entire world to get a few more hundred squared kilometers of dirt? That doesn't make the remotest sense.
Russia keeps the money pit dear to the hearts of many Russians and doesn't become a Chinese vassal state. Ukraine ends the bloodshed and saves their sovereignty for the cost of what they have already lost.
There will be sticking points, for sure, notably around Putin himself, reparations, and yes, the status of DPR and LPR, but I hold out hope that they don't forestall a potential deal.
I can't see the Russian leadership backing down from their stated goals. That means they have to subdue Ukraine, and probably Finland, Estonia and Poland too. If they don't win in at least Ukraine, then they have lost badly, and their economy is destroyed.
I don't think they'll settle now for a piece of Crimea or a piece of Donbas. The entire story is that the whole of Ukraine is a bogus state.
That would be a hell of an escalation. Emphasis on hell. Estonia and Poland are NATO members, which means WW3. And attacking Finland, though not yet a NATO member, would probably invite the largest response from Europe in memory. World war indeed.
Customarily, it's been called a "World War" when both sides have participants from multiple nations. As far as I'm aware, Russia's only ally would be Belarus, which looks to me like a Russian province in all but name.
"WWIII" is a term that also refers to Global Thermonuclear War; if that's what you meant, then I apologise for being pedantic.
[Edit] No attack on Poland or the Baltic states is possible without first subduing Ukraine; so I don't think that's at all imminent. But I wouldn't be surprised if Russia pops a tactical nuke or two in Ukraine, then all bets will be off.
Recognize the annexation of Crimea and promise immediate sanctions rollbacks the moment the last Russian troop leaves Ukraine (other than Donbas and Luhansk, that ship has sailed).