His brother takes over as Tsar
Russia changes a fiar bit
His brother George would inherit only three years after this, but then die before the end of the century of TB (likely a chronic condition in his case and not likely to butterfly out).
Then we get Michael, but will he have gone for a Morganic marriage at that point, or would his elevation in the line end up causing him to find a more suitable marriage?
What about immediate consequences? Anything now different in the Japan/Russia relationship? Could this lead to a Russian punitive mission?
BTW, who would have tried to assassinate Nicholas and have the opportunity to get to a Japanese policeman to have it happen?
And of course, do we get a Russo-Japanese War ITTL? It seems likely we'd either have an earlier conflict over the assassination itself, or none as Michael strikes me as a much more reasonable monarch.