Well there is a small problem with him being assassinated in 1848. He did not assume the throne until December 2. When he assumed the throne he did so at Olmütz which was the headquarters of Fieldmarshal Windischgraetz. He remained there until early May of the following year, at which time he took over control of the military and felt he could not effectively adminstrate from the middle of Bohemia so he returned to Schönbrunn.
Now that means that for the time he was Emperor during 1848, he was basically locked up at the Bishop-Prince of Olmütz's Palace under the guard of a massive loyalist army. Therefore the only way to have him assassinated is to have him killed prior to his ascension which in and of itself raises problems.
Firstly, he was not the first in line to the throne, his father was and there was no reason to believe, at least as far as the public knew, that Kaiser Ferdinand would abdicate, as the last Habsburg to abdicate was Charles V. Therefore why would a would-be assassin aim for him and not his father or uncle. Furthermore, the Imperial Family didn't really leave Hofburg at all during the months leading up to their flight to Olmütz, and when they did leave they left under heavy guard.
Now lets just say for the point of arguement that Franz Joseph was killed prior to leaving for Olmütz, because after that I think it would have been near impossible for any assassin to get near him. You then assume Ferdinand would still abdicate, but with the death of the only real possible heir in the mind of the Imperial Family, Franz Karl didn't want the throne and Maximilian would still be a minor, I doubt that men like Windischgraetz and Schwarzenberg would push Ferdinand to abdicate. So we therefore would like have to wait 2 years for Maximilian to reach majority and therefore for Ferdinand to abdicate. Now in that time, the death of the 2nd in line to the throne would like harden the resolve the of the Reactionaries with in the Austrian government and places like Hungary and Vienna would be brutally crushed. However it would also likely draw moderates away for the liberals because they would be blamed for the assassination. Further I would like to think that you might see areas like Croatia, which remained very loyal to the monarchy because of their dislike of the Hungarians, rewarded with the restoration of their historic rights to spite the Hungarians.