The most obvious POD is a very successful 1848 revolution: if the Habsburgs are in a worse position in late 1848 (say a better performance of the kingdom of Sardinia and the other Italian states earlier in the year, Pio IX not reneging on the liberal concessions and the Croats not saving the bacon of the Austrian emperor) Friedrich Wilhelm might not be so willing to refuse the imperial crown of Germany when the parliament of Frankfurt comes up with the offer. On a wave of nationalistic and liberal enthusiasm, Austria (and possibly Bohemia too) join the new constitutional empire.
May not be as easy as that, but I strongly believe that 1848 was truly a time of possibilities, when Europe might have chosen a very different path from OTL