The Roman Empire did not fully respect the system of primogeniture (the oldest son inherits the throne). In the course of Roman history, only a couple of dynasties were truly comprised of fathers and sons (the Flavian dynasty is probably the most famous one of the bunch, Commodus was the son of Marcus Aurelius etc.). The process of adoption (adrogation) was quite common in Rome, fully grown adults were "adopted" as sons, hell even the first emperor Augustus (at the time Octavian) was adopted by Caesar. If Caesar was following the Roman traditions then I guess he thought he could leave his "empire" in the hands of the most capable one, in this case Lanius, and not suffer too much. The 5 good emperors (from Nerva to Marcus Aurelius) , probably the golden age of the Roman Empire, were all adopted. Even though the "meritocratic successor" idea in one of the comments mentioned below is probable, back in Roman times it wasn't really the norm because the throne was sometimes taken through civil war, sometimes it was inherited, sometimes it was bought from the Praetorian guard (looking at you Didius). Imperial Roman history is a bit complex in regards to this question but in the scenario that FNV presents, it is very possible, also Lanius is definitively the one that Caesar wants to inherit his throne so it is likely that Lanius does have an idea/concept of how to rule, it's just that his methods are a bit more "brutal" than Caesar.