Well, he does look like a Spaniard. I think Maximus saw his son in the afterlife with his wife at the end. Didn't he bury them as well?
I wonder if Connie Nielsen's character, Lucilla, might not have married Septimius Severus in Ridley's alternate timeline? In real life she was assassinated by Commodus after betraying him (he was not so merciful in real life).
If Septimius is of higher nobility would his and Lucilla's boys then not be first in line for emperorship? How does ancient Roman ascension work?
Or perhaps Caracalla (next in line) connived to sell Lucius into slavery in North Africa removing Lucius from the ascension line and subsequently causing the rift between Caracalla and Geta.
After Geta gets offed and Caracalla gets offed in revenge, then perhaps her older son, Lucius as sole surviving heir to the Empire needs to step in to clean up the mess like
@KiramidHead speculated?