I think upgrading existing pre Marian troops to their post Marian counterparts is a little too much. In reality, there was a lot of opposition to the Marian reforms. The Marian reforms came about because of a huge lack of manpower - the able bodied, propertied men are precisely the ones you want to have at home, plowing the fields and attending to their businesses. So Marius recruited from among the Head Count, the underprivileged and jobless, people from the lowest rank of society. Because of that, the Senate wanted nothing to do with these troops, and there were still many armies using the older style of troops for many years after. I think the game as it stands now portrays it more accurately - as soldiers in the old style armies serve out their time or are killed, the new style armies gradually replace them.

I believe the last great old style army assembled was for a battle at Arausio. The Romans lost miserably because of a lot of wrangling between the commander appointed by the Senate, and the guy who thought he ought to be commander... after that, there just weren't enough able bodied, propertied men left in Rome OR Italy to do much. (The Senate had to call on Marius, whom they loathed, to again assemble a Head Count army to avenge that defeat and stop the Germans invading Rome...)

I like the levy idea! That would mean that the Barbarian factions would have far mroe of a role playing feel to them, because you would have to train up your leaders and really watch their V&Vs...

I would like to see a more variable AI (when they get the basics sorted out). When I face an army lead by a "good defender", it should behave differently from one led by a "good attacker" (or a "social drinker", for that matter).