National Post reports " ... One focus of the build-up appears to be the port of Mariupol, the second biggest city in Donetsk with a population of 500,000. Mariupol was under separatist control until last August when it was retaken by Ukrainian forces. The city lies on the E58 highway running between the Russian border and Crimea, the region annexed by the Kremlin last year. Capturing Mariupol would help Russia to seize an overland link to Crimea. Ukrainian sources claimed that rebel artillery and tanks fired at government forces in Shyrokyne, a village on the front line near Mariupol, on Wednesday.
The US said Russian troops were also massing 300 miles further north in the border area opposite Kharkiv, a Ukrainian city that has remained under Kiev’s control throughout the crisis. The latest military movements suggest that Russia’s aim would be to threaten Kharkiv and Mariupol simultaneously...."
Also this article from Newsweek further elaborates on Kharkiv and on Odessa far to the west. http://www.newsweek.com/odessa-holds-its-breath-invasion-324794 It suggests May 9 Victory Day could be critical in Kharkiv and May 2nd in Odessa as an anniversary of a major killing last year.
... In Dnipropetrovsk, which is 218 miles southwest of Kharkiv and 200 miles west of the front lines), the recent departure of the city’s mayor, oligarch Ihor Kolomoisky, has residents on edge. Kolomoisky is credited by many with preventing a separatist takeover last year.
Russia's navy could strike Ukraine from the Black Sea, but it doesn't have enough manpower to launch a large-scale landing. That is according to the Chief of Staff of Ukraine's navy, Rear Admiral Andriy Tarasov.
Rear Admiral Andriy Tarasov, Ukrainian Navy: "The first thing the airborne assault brigade has to do is to conquer the sea. The second thing to do is to conquer the airspace and in order to have the opportunity to arrange ‘surprise' attacks. If one of these tasks is not fulfilled, the airborne assault operation will inevitably fail. This is a very difficult type of operation. Of course, troops are most vulnerable while crossing the sea. They have no protection at sea, so that they are fully open during landing."
Ukraine has been busy bolstering its defences. The military has planted mines near the coast on the Black Sea to prevent the Russian equipment from advancing on land...." http://uatoday.tv/geopolitics/ukrainian-navy-says-russia-could-strike-from-the-sea-418531.html
I'm certainly no expert but if these troop concentrations are correct then the road E105 south from Kharkiv and the road M14 west from Mariupol will be key and they link at Melitopol on the route to the Crimea.
starts from Hesseng, (just south of Kirkenes), Norway and runs along Russia's M18, M10, M2: Ukraine's M20, M29, and M18 to Yalta, Crimea (disputed territory). Russians call this the Crimea Highway (Крым шоссе). - Wikipedia http://en.wikipedia.org/wiki/European_route_E105 This is a 2870 km long road. Of some interest and in passing note, the Norwegians have declined to complete the bridge between Russia and Norway of the E105. Or at least it is delays for several years. http://barentsobserver.com/en/borders/2015/04/norway-delays-bridge-building-russia-road-crimea-23-04