Here's my speculation about a suspected/possible line that could be hold outside of the currently contested oblasts of Lugansk and Donetsk.
Pic #1 borders, just to see where they are.
To the west of Lugansk and Dontesk are Kharkov and Dnipropetrovsk oblasts. To the south west - torn in two - is Zaporozhia (or Zap as Matt of WillyOAM calls it), further south west is Kherson, then Mykolaev to the west and in the end Odesa to the very west, not even on this map.
Pic #2 the northern part is fairly simple, within Kharkov. Kupiansk, Izium, and Barvinkove are the major strongholds. The Oskil river runs here, down to Izium it has a fairly obvious contour. Around Barvinkove there are some topographical features that can be used, still a hilly region, with some brooks running all over.
The problem is towards the south.
Pic #3 is the possible strongholds and rivers.
Optimally the AFU should want to hold Sloviansk, Kramatorsk, Pokrovsk line - green dotted line - there is a river between Druzhkivka (south of Kramatorsk) and Pokrovsk (underlined with red in the south) too, with some hills along it. But if they are giving up Donetsk, this won't be available.
Between Barkivkove (NE green) and Petropavlivka (SE green) there is a brook and a river, but from Oleksandrivka (N red) that line might be vulnerable and perhaps can be circumvented with a thrust to Lozova (NW green). Lozova's perimeter is a good candidate to fortify, and a river is going down south to Pavlograd (SW green). So here there is a double line that gives itself, and some more brooks and hills inbetween.
In the Pokrovsk - Pavlograd direction the terrain seems more flat, open and no real good place to hold near to the border, except to the very south with a consequent lines of rivers (and more to the west). Along the road, especially on the south side of it, there is not much to hold onto.
From Pavlograd to the west, the terrain seems a bit more difficult, therefore easier to hold.