Okay. As a lot of people here know, Peter III of Russia, when he came to the throne after Elizabeth's death in 1762, was highly unpopular for his pro-Prussian policies; as always happened with unpopular rulers, there were many plots against him, one of which ultimately succeeded several months later.
IOTL, after his death the throne went to his wife Catherine II, and eventually to their son Paul, from whom all further Romanov rulers were descended.
POD: one of the plots against Peter in 1762 succeeds in killing Paul (either deliberately or accidentally), so he is dead by the time his father is killed, and isn't available as the potential successor further on.
I assume Catherine still inherits as OTL; question is, who is the heir/crown prince/first in line? Genealogy really seems to say that it would be either (still imprisoned) Ivan VI, one of his (also imprisoned) younger siblings, or, if that's entirely not an option, this guy; neither possibility is likely to be anywhere near popular (though Catherine might well like the last one).
IOTL, after his death the throne went to his wife Catherine II, and eventually to their son Paul, from whom all further Romanov rulers were descended.
POD: one of the plots against Peter in 1762 succeeds in killing Paul (either deliberately or accidentally), so he is dead by the time his father is killed, and isn't available as the potential successor further on.
I assume Catherine still inherits as OTL; question is, who is the heir/crown prince/first in line? Genealogy really seems to say that it would be either (still imprisoned) Ivan VI, one of his (also imprisoned) younger siblings, or, if that's entirely not an option, this guy; neither possibility is likely to be anywhere near popular (though Catherine might well like the last one).