Austria, Germany, Britain, and France all had inventors/innovators propose the development of the tank in the early 1900's. All were ignored. I have perfect hindsight vision, but I still have to wonder why. It was obvious that machine guns and modern small arms were going to be a deadly defensive weapon. Armoring personnel or their carriers does have a precedent, so the advantages should have been seen, and effective light/mobile tanks/troop carriers/munition carriers developed. Still, Germany ignored the advantage of steel canon until Krupp fortuitously gave one he couldn't sell to Wilhelm I and he demands it be given a shot. You could have Kruppwerks repeat the scenario with tanks. Once the advantage of tanks were seen, development was initially iffy because of the immediate speed in which they were needed to be fielded. Slow that down to reasonable development speed requirements, and it's easy to field a tank in WWI. All that was lacking was the will, and that's an easy magic to create (see Rast's TL)
Give that scenario, trench warfare is either avoided, or ended early because heavier tanks could have been easier to develop off the basis of the earlier tanks. If only one side has tanks, it's game over for the other side. If both sides have them, with all else staying the same, I don't know enough of military tactics to say who has the advantage. Assuming that Schlieffen plan remains, Germany can sweep through quicker, but one would also think that France could breakthrough in the center rather than running into a brickwall, and the S plan required France to get stonewalled.