1. Conflict subsides, DNR and LNR become what Colonel Cassad scathingly calls "Greater Transnistria", with a destroyed economy and an impoverished populace. Unlikely, since all possible diplomatic negotiations between the opposing parties have already broken down, the fighting is constantly intensifying and nobody, especially the United States, is willing to accept a compromise.
2. Conflict flares up and dies out a few months later with all sides exhausted. The possible results are:
a) No significant gains by either side, end outcome like point 1, unlikely since the United States and the EU are not going to accept an independent DNR and LNR;
b) Ukraine defeats the DNR and LNR, probably with NATO help. The worst possible scenario is Ukrainian nationalists expelling the Russian-speaking "bydlo" inhabiting the "Donbabwe" and "Luganda" like how the Croats did away with the Serbian populace after the fall of Republic of Serbian Krajina in 1995,
though I'd really like to see the democratic media whitewashing that.
Ukrainian victory is rather likely to me, way more likely than DNR/LNR victory in any case.
c) DNR/LNR win and expand their territory, probably with the help of Russia. Rather unlikely, since Putin has been constantly trying to end the war with a compromise leaving the DNR and LNR as tiny weak spherelings like Abkhazia. Some say he is afraid that Donbass veterans like Strelkov can become a viable political alternative in Russia and challenge his grip on power, and a decisive DNR/LNR victory can potentially boost their political weight, so he attempts to prevent that.
3. Russia actually invades Ukraine, boots Poroshenko out of office, replaces him with a puppet. Extremely unlikely - Putin is way too indecisive for that.
4. Russia invades Ukraine and later on attacks Lithuania, Latvia, Estonia, Poland, Finland or some other Eastern European country for some incredibly unclear and incredibly evil reason. Extremely unlikely - it's a guaranteed WW3 and it won't help Putin stay in office.
5. NATO decides to launch a pre-emptive (or "pre-emptive") attack on Russia and its allies - way more likely than a Russian invasion. The real and perceived tactical and technological superiority of NATO forces can make people in charge of the US think that a short conventional war against Russia would solve their political goals (eliminating a potential threat to American hegemony) without any significant political, economic and military damage. Unlike them, Russian high command is aware of Russia's weaknesses against NATO, so a Russian first strike is very unlikely.