The Center for European Policy Analysis (CEPA), a nonpartisan, prodemocracy think tank based in Washington, D.C., recently undertook the ranking of "potential scenarios and implications for medium-term European security" in the context of the Russia-Ukraine war. CEPA's conjectures are reproduced below, with only mild editing. 
The think tank ranked the outcomes — seven of them — from the potentially worst to the optimistically best. Here, I have reordered them, leading with optimism and concluding with the absolute worst outcome. I would have reversed CEPA's numbers 1 and 2 for two reasons: retention of the Donbas (ranking #2) would be far more valuable to Ukraine, economically and spiritually, than the permanent loss of Crimea. In addition, Putin's ouster alone elevates the #2 ranking to the #1 spot, especially with Russia "giving way" to a renewed democratization.
Also, CEPA's #4 and #5 could easily, and logically, be combined (Russia's destabilization being their only real difference), though I have left them separate.
Finally, omitted from mention in #7 is Russia's military incapability for a considerable stretch of time — perhaps never; not, anyway, as long as Russia remains an international pariah subjected to withering sanctions on military technology, supplies, parts and the like.
CEPA's rankings:
1. Ukraine wins. Russia is forced to withdraw its forces from Ukraine, except the [Donbas regions] which retain – for the time being – their ersatz 'independence.' Ukraine gravitates towards Europe and, in time, achieves EU membership or candidate status. NATO membership remains a theoretical, but distant possibility. EU and NATO influence expands not only in Ukraine, but also in other ex-Soviet republics – the Caucasus, Moldova, and even Belarus. Moscow loses influence in the Balkans in the wake of Putin’s defeat....
2. (Almost) everyone is a winner. In this scenario, Russia pulls back from most of Ukraine, including the [Donbas], but retains Crimea. There is a new international treaty enshrining Ukrainian sovereignty and formalizing agreement on major territorial, security, and political questions. In Russia, Putin leaves office, giving way to a (somewhat) more liberal leadership and a renewed if imperfect process of democratization. President Alexander Lukashenka is ousted in Belarus, and the beginnings of a nascent and fragile democracy emerge there. Despite these favorable trends, a 'single Europe' would remain unlikely. Although the prestige of EU and NATO would be enhanced, Europe would be a 'multi-speed' continent – politically, economically, socially, and culturally diverse. Crucially, though, these differences would be broadly manageable while the chances of conflict within Europe would diminish.
3. A Ukraine divided. Russia annexes the rest of the Donetsk and Luhansk regions, building a land bridge to Crimea. It may even shut out Ukraine from the Black Sea by taking Odessa.... Ukraine would retain its sovereign independence and move closer to the West. The consequences for Europe would be similar to scenario two in that the continent would be divided, only the line would be further to the east. Under this scenario, the Baltic states would remain within Western security and economic structures, as would Bulgaria and Romania (and Turkey within NATO). This outcome, however, is scarcely sustainable. Moscow would not relax its efforts to subjugate – directly or indirectly – those parts of Ukraine outside its sphere of influence, while seeking to extend its remit to the Baltic states, Moldova, Georgia, and the Balkans.
4. Stalemate/low-level conflict. This scenario would essentially be a continuation of the status quo ante the 2022 Russian invasion. Putin claims 'victory'; he pulls the troops back from most of Ukraine but reinforces the Russian position in Crimea and the [Donetsk and Luhansk] regions. There is continued sporadic fighting, similar to that during 2015-2021. The main danger in this scenario is 'rinse and repeat' – a return to the complacency and indifference that have characterized European attitudes towards Ukraine in recent years. Basic failures of governance in Ukraine would remain unresolved. There would be a pause, not an end, to Russian attempts to dominate (or disrupt) much of Eastern Europe. And plenty of scope for renewed conflict down the line.
5. A long grind. In this scenario, the war in its current form would go on for years, with each side having wins and losses along the way, but no decisive victory for either side. The result would be a near perpetual state of conflict on the continent with the constant threat of further escalation hanging in the background. Casualties on both sides would continue to mount. The West would continue to provide support to Kyiv, yet the status of Ukraine and its ties to Western institutions would remain unresolved. Meanwhile, domestic pressure on the Kremlin would increase, similar to what the Soviet Union experienced during its long war with Afghanistan, risking destabilization inside Russia.
6. Russia wins. Under this scenario, the Ukrainian armed resistance is decisively defeated, and a Russian puppet government is instituted in Kyiv. A rump Ukraine retains nominal independence but submits to Moscow’s political and economic fiat while joining Russian-led structures, such as the Collective Security Treaty Organization.... Europe would again be divided into two armed camps as at the height of the Cold War. Russia’s de facto acquisition of Ukraine would have a chastening effect on other ex-Soviet republics. There would be direct security repercussions for front-line NATO member-states such as Poland, Romania, and Bulgaria, and the three Baltic states would be especially vulnerable.
7. Wider European war. Putin’s willingness to escalate the conflict in Ukraine significantly increases the potential for conflict between Russia and NATO. This encompasses a range of possibilities: from relatively low-level skirmishes in and around NATO front-line members (the Baltic states, Poland, Romania, Bulgaria, Turkey, Norway) to a major European and possibly even transcontinental war involving not only conventional armaments, but also chemical, biological, and nuclear weapons. In the most drastic scenarios, the conflict between Russia and the West would be existential. There would be little point in speculating about the future of Europe until this is resolved in one form or another – either with a decisive defeat of Putin’s Russia, the collapse of the US-led West (and of the post-Cold War settlement), some sort of negotiated settlement along the lines of Yalta 1945 (or possibly the 1953 Korean armistice), or nuclear Armageddon.