Hmm, good question.
The Cantabrian Wars is an excellent DLC but it is tough to play on both sides. And you must be updated to the latest versions for it to be balanced. But it is a single theater with focused play.
The Spartacus Revolt is a very decent choice. Spartacus is a tough opponent. I do not know if you have read Mary Beard's excellent book SPQR: A History of Ancient Rome where she asserts that many of Spartacus's men were ex-legionnaires of Sulla's armies. This scenario respects Spartacus' strength in Beard's sense of his power, not just slaves, but men asserting their worth.
So maybe that DLC?