In the meantime, any of the small skirmishes that have never ceased to occur along the Dombass front line will serve as an excuse for Russia to claim that Ukraine has begun a large-scale operation to recapture the Dombass (and Ukraine's massive military presence there right now, to prevent an invassion, will make it easier for Russia to justify).
Russia will then say that it has no choice but to enter with its army to help the population of the Dombass republics, and that it has the legal right (and 'moral obligation') to do so, since they are in the process of becoming an official part of Russia (and in fact the bulk of their population has already a Russian passport: Russia started some years ago issuing passports there to anyone who wanted them).
Russia would not limit its offensive to the Dombass area but will attack Ukraine along the entire frontier, with emphasis on the border area closest to Kiev, not with the aim of taking the city (it will not do it, nor will it try, it will only get close, as it did with Tbilisi in 2004).
It is possible that it will also attack from the south, from Crime, in the direction of Odessa.
The purpose of occupying patches of territory in Ucrania well beyond Crimea and Dombass, and in the proximity of Kiev, will not be to retain them afterwards, but to have the upper hand during the negotiation process later.
I believe that it will be a very short but very bloody war, with obicous use of heavy and sophisticted weaponry. It is possible that the Belarusian army will also intervene directly on the side of Russia. Neither the US nor the European countries will send ground forces, although mercenaries and volunters will suport both sides and there will be a large supply of weapons to Ukraine from the West.
The war, although very short, will provoke a brief global crisis with a crash in the stock markets, a brutal rise in the cost of energy and massive hacker attacks on Western interests.
Everything will be very, very fast. In a few days, and with Russia having gained control of portions of Ukraine, but without having at all 'conquered' most of the country, not even getitng close to do so, there will be a ceasefire intermediated by countries neutral in the conflict, such as Turkey or Sweden.
A formal negotiation process will strat, in which Russia will agree to withdraw from all the territory of Ukraine occupied by its forces, returning to the pre-war situation (Crimea and Donbass under Russian control) in exchange for (1 ) the international community declaring and granting a 'neutrality' status for Ukraine (such as Finland's neutrality agreed after WWII) so that Ukraine will never join NATO and will become a buffer zone for Russia and (2) the international community recognizing the status quo of Dombass and Crimea as part of Russia. I think Russia will get (1), but probably not fully (2)- perhaps the fate of those territories will be tied to some future UN-granted referemdum .There will be international ground forces deployed to observe the ceasifire and the agreement, under UN aegis.
Hence, I don't think it will be a long conflict, neither that a total annexation of Ukraine by Russia will happen, nor that a world war IIII will erupt. But the problem with violent conflicts like this is that, altough we may predict more or less how they strat, there is also the risk of things later to go quite out of control, so eventally things might turn even nastier...and there quite many ways for that to happen in this conflict (e.g. the Baltics geeting involved, or Molodova melted on this because os Transnistria, etc...nor to mention China taking advantage of the situation and going after Taiwan). But I don't think any of these further 'spill overs' will happen. The war will be confined to Ukraine.
Lets hope that even this 'limited war' scenario I am predicting will not happen and that peace will prevail. But I am not very optimistic.
At the end , as in every conflict, those that will losse the most will be the civilians: the women, men and children trapped in the middle of the violence driven my high political and economic interests. Sad.
