As early as 1903 IIRC there were plans for motorized vehicles with large guns, so it's not totally unrealistic. The main problem is that the militaries of Europe were very conservative, as in they preferred the old ways of waging war (an example is how many believed tanks to be exclusive to WWI and everyone would go back to using cavalry afterwards). If you could find a way to bypass this mentality, then sure, tanks could be used maybe as early as the Italo-Turkish/1st Balkan War.
Probably the best way to bypass this mentality is for it to be seen as a piece of engineering equipment, something used for attacking fortresses, the engineers I would imagine are less conservative and something for reducing fortresses faster fits in with the dominant maneuver warfare paradigm.
To do this primary armament would likely be a flamethrower, to burn out outlying works that would otherwise require some of the huge siege guns to deal with. Tracks would be to cover ditches, fascines and other obstacles, while armor would protect against rifle fire, and you would want to mount machine guns of your own for protection. Speed isn't really a priority, but greater mobility than something like Big Bertha or the Gamma Morser is, because you don't want your army waiting around longer than it has to
As a piece of engineering equipment the logical developers would be the Germans, as they went heaviest into equipment for taking fortresses as they planned on having to go through Belgium and its fortresses. Impact would not be too much, but say Liege falling on the 15th instead of the 16th while small generates enormous butterflies, more likely to the Central Powers benefit than not
