The 30 Years War, horrific as it was, could certainly have been even worse. The 'easiest' way to make it worse probably is to extend the geographic area it covered. An obvious move would be to have the Deluge (1648-1667 Poland) happen twenty years earlier, so that it coincides in time with the more destructive second half of the 30 Years War. On a per capita basis, the Deluge was at least as bad in Poland as the 30 Years War was in the Germanies (estimates are that the Deluge resulted in a Polish population loss of about 30-35%, even greater proportionately than the 25-30% that the Germanies had suffered). The Swedes were heavily involved in both blood-lettings as well, which works both for and against the two conflicts happening at the same time -- they might be too ever-extended to sustain heavy fighting in both regions. Although one reason they were so destructive to the population was their practice of living off the land, at the lethal expense of the inhabitants (Pomerania, for example, which was occupied by the Swedes for almost all the 30 Years War, suffered up to 70% population loss during that time), so perhaps the Swedes could have managed it.
On a side note, I would not be surprised to find out that the Scandinavians (Swedes and Danes both) caused more death and misery, on an absolute basis, during the years they were involved in the Germanies and Poland (1625-1657) than they did during the entire Viking period. Not that they were the only culprits by any means.
Another terrible, destructive war that was loosely connected with the 30 Years War was the War of the Mantuan Succession (1628-1631), which according to the one estimate I have read, killed about 25% of the population of northern Italy. Again, some of the same Powers (France and the Hapsburgs) were involved in this war as in the 30 Years war. It already coincides in time with the 30 Years War.
Off the top of my head, England, France and Spain would also endure grinding civil wars, religious conflict and struggles for independence in this same time period. I would think that it would not need too much for all these conflicts to end up part of a generally recognized Great Pan-European War.
So basically, it would not have been impossible for the 30 Years War to turn from a mainly German war (at least from the point of view of the civilian casualties, the combatants were from most of western Europe) into a pan-European war, with similar population losses as were suffered in Poland, northern Italy and the Germanies in OTL, but even more wide-spread.