I went with a transitional presidency. I wholeheartedly agree with the 1896 parallel especially if Warren winds up as VP.
Like President William McKinley, a President Joe Biden dies in office early in his term with his Veep serving the remainder (although not as a result of an assassination). I hope that whoever serves the role of a Theodore Roosevelt shouldn't be as squeamish as he was about effectively serving two terms but only running for one in his own right.
If the Presidency feels vacant with more than two years remaining, it counts like a full term for the VP. Ford wouldn't be eligible to run in 1980, while LBJ was in 1968. Truman was President when the amendment was passed, but he was still eligible for 1952 due to grandfather's clause.
During TR's time there was no term limits whatsoever, just Washington "precedent".