lets say, 70%ish of the European population (while their diseases decimate the Native population). How would that change world history?
70% ish is very optimistic for a disease. The deadliest contagious viral diseases (which seem to be what you're describing) like smallpox seem to generally kill about 1/3 of the population in a virgin epidemic, all else being equal, and can
kill less than that if a population has the option to scatter (which Europeans west of the steppes probably wouldn't).
The massive drop in Native American population came as a result of a mix of disease, violence and dispossession which prevented their numbers from recovering. Disease alone can crater Europe's population, but depending on the disease will not do it for long. Depends on the epidemiology, though-a Western Hemisphere yellow fever or malaria that establishes itself in mosquito or sandfly populations around the Mediterranean, for example, could really fuck with the human population there and see large amounts of land become and stay empty for centuries. A smallpox or measles equivalent, on the other hand, would burn through everywhere but would leave behind immune individuals who could recover over the course of a few generations.
That said, assuming this is an early modern and not a Norse introduction of the disease, then Europe is being hit at a sensitive time. Tensions between different monarchies and social and religious movements are ripe and about to explode. A disease that hits Europe, let's say transported back with Cortes just for shits and giggles, is going to leave in its wake a whole bunch of millenarian movements, including alt-Protestant and uber-Catholic movements reacting to them, with a 'moderate' Catholic ideology trying to keep the status quo for the nobility and church mainly by burning as many disease survivors as possible at the stake.
The resulting wars could see Europe's population fail to recover from quite some time, as violence, famine, and native diseases like typhus and typhoid fever run rampant in addition to the New World plague. IOTL, some of the diseases that affected Native Americans like cocozotli (sp?) may have been indigenous diseases, which just had good conditions to spread at a very bad time in the wake of the conquista.
So doomsday scenario is a smallpox equivalent kills a third of the population in mainland Europe, and then for most of Central Europe, that population drops further as a very nastier version of the Schmalkaldic wars occur, with concurrent wars in Scandinavia, Britain, France and possibly Northern Italy over the next several decades.
This will, I think, delay mass European colonization of the Americas for quite some time beyond a few autonomous colonies with European elites ruling over Native subjects. A re-run of the aftermath of the Black Death sees wealth and opportunity increase after the wars, with vast tracts of land open for the common farmer to make money off of, generating wealth for the common people and artisans in urban areas. A reassertion of the social hierarchy will see elites try to keep control by limiting emigration to the Americas-either way, the Natives have a little breathing space, and while I don't think it will be enough to avoid colonization altogether, I think we could see independent Native nations surviving contact ITTL, with this world's equivalent of the Inca and Maya being especially good candidates for keeping independence.