These are the Roman Camillian Consular armies I was using in a recent game which I'm looking forward to recreating in a longer campaign in the future. Influenced by a mix of the official suggestions from this thread, and stuff I otherwise read and tried to recreate accurately. Using manipular formations in battle with the classic three lines + skirmishers in front, with allies on the wings (alae meaning wing after all). I'd thin out the lines into four ranks just before engaging to cover a bit more distance since there would only be four units per line, and with the default eight ranks it was too easy to be outflanked and overwhelmed.
Battles would tend to end up with the second line moving up to fill in gaps and overlapping to double strengthen lines (essentially back to eight ranks) sometime after the initial clashes, and third line units might even be needed to cover flanks as needed. This part might not have been historically accurate, as my understanding is the second line would move up after the first line broke or as it was backing up, and the third line was rarely used at all, but the practicalities of Total War battles didn't really allow for that. Besides, what would usually result were some beautiful battle lines anyway and I can never get enough of those for screenshots.