If the HRE centralization efforts of the Hohenstaufen pay up, "Germany" (actually a binational German-Italian state) would end up to include the Low Countries, Switzerland, Austria, Bohemia-Moravia (no doubt Germanized over time), Italy (northern Italy was part of the HRE, southern Italy got in dynastic union and would be absorbed). Moreover, no doubt the Germanization of Poland and Hungary (actually, quite likely it would be mixed Germanization/Italianization for Hungary, Slovenia, and Croatia) would be much more successful than OTL. We could easily end up with the eastern borders of the HRE on the Vistula and the Carpathians.
Moreover, as it concerns the western border, a strong unified HRE would at the very least put a mighty check on the eastern expansion of France. The Franco-German border would remain on the Maas/Meuse and the HRE would keep Alsace, Lorraine, and the Franche Comte.
I personally think that if Italy is included in any super-germany/Holy Roman Empire, That it would eventually stop being German and revert back to Roman in name at least. But the culture would be definetly Germanic as Northern Italy was really since the Lombards settled there. So a Germanic speaking Western Roman Empire. I can see Germanic being spread around Europe the Way Latin was. Now that would be quite interesting in my honest opinion.