Through archives and doctrinal publications, was there any bias towards attacking NATO through the North German Plain, the Fulda Gap, or something else?
Two declassified Warsaw Pact documents give insight into their plans. The Warsaw Pact "Plan of Actions of the Czechoslovak People’s Army for War Period" from 1964 which outlined the plan for Czechoslovakia in the event of a NATO attack and Seven Days To The Rhine which is a response to a NATO nuclear first strike to cut off Eastern Europe from the Soviet Union.
The plans make several things clear: The Warsaw Pact expected NATO to strike first with a tactical nuclear strike, and they saw a limited nuclear war as feasible. This is the opposite of NATO's own planning which assumed the Warsaw Pact would strike first, planned to avoid nuclear weapons if possible, and planned to respond to nuclear weapons with a disproportionate response.
The Czech plan has them attacking through southern Germany. The Czech's were to be prepared...
To be ready to start advancing toward Nuremberg, Stuttgart and Munich with part of forces immediately after the nuclear strike. Nuclear strike against the troops of the enemy should be targeted to the depth up to the line Würzburg, Erlangen, Regensburg, Landshut.
Yes, they expected troops to march through territory they had just nuked. The Soviets tested this with 45,000 troops in 1954.
The immediate task is to defeat the main forces of the Central Group of the German Army in the southern part of the FRG, in cooperation with the [Soviet] 8th Guards Army of the 1st Western Front; by the end of the first day – reach the line Bayreuth, Regensburg, Passau; and by the end of the second day – move to the line Höchstadt, Schwabach, Ingolstadt, Mühldorf, and by the fourth day of the attack – reach the line Mosbach, Nürtingen, Memmingen, Kaufbeuren.
In the future, building upon the advance in the direction of Strasbourg, Epinal, Dijon, to finalize the defeat of the enemy in the territory of the FRG, to force a crossing of the river Rhine, and on the seventh or eighth day of the operation to take hold of the line Langres, Besançon.
Afterward develop the advance toward Lyon.
This has the Czechs tearing through southern Germany in a week then sweeping south through France along the Swiss border.
Seven Days To The Rhine assumes NATO has cut off Eastern Europe with a tactical nuclear strike at key crossings of the Vistula River. Pact forces already in Eastern Europe would counter-attack West Germany, Belgium, Netherlands and Denmark necessitating several lines of attack, not just central Germany.
Unfortunately, I cannot find a copy of the plan.
The best book that I have seen written on the subject by a writer who should know is: The Third World War, August 1985 by General Sir John Hackett. Hacket is a retired British Army general who published this fictional account in 1979. See: http://www.amazon.com/gp/product/0425101924?keywords=The%20war%20in%201985%20Hackett&qid=1445134086&ref_=sr_1_2&sr=8-2
I am sure those plans would be quite secret and you would not be able to trust any offhand account of them because there would be no telling whether the informant had correct information or was reporting some kind of disinformation.
In general, all countries make elaborate military plans and strategies of diverse kinds for all different situations and contingencies, so there is no one plan. Of course, when an attack is actually made a plan is chosen, but before then no one specific plan will exist, but there will be many different plans.