Personally I'd like to see some spin-off series ; and here are my ideas :
1. A Gannicus "prequel" to "Vengeance" ; about his life while in freedom ; he would probably be a gladiator for hire in the whole Roman Empire ; lots of fans complained about the lacks of "arena" in the last seasons ; it would be a good way to bring back some gladiatoral games.
2. An Oenemaus background : I would really like to see a deeper look at what was shown in Vengeance from his arrival into slavery to the pits to his recruitement by the old Batiatus ; his fight against Theokoles, etc ... It would be great to see the former Doctore being a bit expanded too.
3. A background about Batiatus' father/grand-father : I would love to see his "champions" expanded a lot ; maybe even on his parents too ; to see Magneteus, Acerbitas, Adrianus, Dolor and Zephiros ; also the former Doctore (Ulpius) and Oenemaus in his youth.
4. East of the Rhine backgrounds : maybe a story about the backgrounds of the germans warriors before their enslavery. I'd like to see Lugo, Saxa, Nemetes and Sedullus being expanded too.
5. Ashur's kill squad background : I'd love to see some short stories about Ashur's men ; how he knew them and what they were doing before. Most of them should be in jail or might be free gladiators and excepted the Egyptian ; they were all free before Ashur recruited them.
6. Roman spin-offs : I don't like much the idea of a plot in Rome with Caesar and Crassus ; excepted if their opponents are from somewhere else (War of the Gauls for Caesar) ; I don't know much about Crassus follow-up histories ...