Donbas

Donbas is Ukraine eastern most area. It was settled and developed during 19th and 20th centuries. It is heavily industrial areas with steelworks and coal mines. The area is heavily populated as well.

Back when mining coal in mines were still profitable the area was very prosperous and was even called heart of Russia.

Donbas never knew anything, but heavily polluted air and prosperity until open pit mining made mines obsolete.

It is like West Virginia and Detroit combined, but worse. Its mining population as willing to blame everything on Ukraine as UK miners are still willing to blame mines closure on Thatcher government.

This economically depressed region is with a huge population it is like a burning powder keg waiting to explode. 

So, from economic perspective it was very fortunate that these areas decided to secede from Ukraine and become independent nations.

Geographically it is just like Kharkiv very close to Russian border. Across the border there are also mining town just like those in Ukrainian Donbas.

Economically this area is heavily dependent on Russia as most of its inhabitants work in construction in Moscow and bring home remittances from this work.

Because of this fact this area is important for Russian economy but very detrimental for Ukrainian one.

Donbas Republic

Miners on each side of the border has sympathy for fellow miners. They all suffer from the same economic ailment and mining character of this area serves as an identity fot the area. Thus, it will make sense for them to unite the Donbas area into one nation that can be called Donbas Republic.

Though areas north of Donets River should go to Sloboda Ukraine Republic instead.

Territory can be made even bigger like that of the civil war era Don Republic

This can become another viable new buffer state on the borders between Russia and Ukraine. Economically it will be dependent on Russia though.

This area is somewhat more important geographically as it includes isthmus of Don River. However final borders of the Republic can exclude the istmus and cities like Rostov.