It's recorded that during the first two years of his reign, Caligula was a noble and moderate ruler. As well-known, he had a break down and became Caligula as we know today.
Now, let's say he avoids drinking so much wine (avoiding the possible lead poisoning) or manages to keep his problem under control. What sort of ruler would a reasonable Caligula be like? Without his assassination, who would be the appointed heir?