I'd second the recommendation of RTR 6.0. One thing it does, I think, is make pretty much all units immediately recruitable from a faction's capital. (It also deletes peasants and town watch). It also pumps up some of the non-Roman factions. As a result, you face more and better AI armies. In my Roman game, the Gauls early on had a smattering of "elite" units as did the Greeks. My later contests with the Ptolemics and Seleucids regularly featured stacks of mainly elite phalanx and shieldbearer (hypasists or some such name) units.

It also helps to get a challenge if you limit your own forces. As Romans, my stacks would be half allies and only one or two slinger/archer units.

Obviously, play on VH campaigns if you want stronger AI armies.