Perhaps Maharal's greatest contribution was his innovative approach to interpreting the Aggadic portion of the Talmud...
Maharal's innovative approach was not merely in the way that he interpreted Aggadah - as metaphysics rather than metaphor - but also in his applying this approach to passages in the Talmud that were not formerly interpreted as Aggadah. A striking example is the dispute on Pesachim 94b, which before Maharal was never interpreted as anything other than a dispute about astronomy.
In sharp contrast to this view of Maharal is the view of Maharal that emerges from the teachings of another creative genius: Rav Moshe Shapiro, as reflected in Chaim B'Emunasam. According to this view, Maharal's approach was not innovative in the least. Rather, the Rishonim always interpreted Chazal this way and only this way. Maharal was merely the first one to put it into writing.
(If you haven't yet read Rabbi Chaim Eisen's superb essay on another innovation of Maharal - his elevating Aggada to dogma - as well as how Maharal did not become popular until recently, read it now!)