I admit - I’d rather go semi the other-way entirely.
Instead of playing a single, mighty unified empire, you’d play one family (as before). You build up some alliances, but the core of your mini-empire is trying to grab control of subsections of counties. (provinces. whatever.) Sure, you have troops and use them, but you’re trying principally to build up your power base so you can take over the empire, peacefully or otherwise, from within. At the start, you couldn’t even control a whole province or get taxes from it: you’d have to take missions, build up economic alliances and strongpoints, and then eventually declare yourself dictator.
War would be something relatively quick compared to previous games. In the early game you wouldn’t even be able to afford a standing army (no taxes to pay for them). Instead, you’d have to save up, grab troops when you need them, and then disband them as soon as you could. In the meantime, you would hope to grab some more economic influence and wartime glory. If you wanted to keep a alrge army in the field for a while, you’d probably want to despoil a lot of territory…
…like, I don’t know… Gaul might be a good choice. 
This would also be a neat bit of self-balancing to challenge the player. If the Empire has given you a military mission, you’ll probably get an army to use and can stack any troops you have as well. So the game can help out strggling players with a couple slowballs. On the other hand, if you try to expand you own power base, it can give you some tougher missions to occupy your time (as “The suspicious Senate is keeping you occupied/wants you to fail/whatever”. And of course, any military actions for yourself will be entirely on your own, whereas the enemy has taxes to pay for their own ongoing defense and can probably outlast you. So you have to strike really hard and fast to bring them to their knees, possibly with an undermanned and underequipped force, and even then you probably aren’t going to conquer a whole state.
In concept, you’d move up in rank. First you’d be a private citizen with a small powerbase. Then you’d get some lower ranking Roman titles and the chance to buy popularity. Eventually you’d want to become a Senator and form alliances with the power players in the Republic. Then you’d hopefully become Consul, and then ProConsul governor of a province or get a major military command. And then you might just be strong enough to take on the other factions of Rome and win.
Or at least, that’s the crazy dream I have. Call it Europa Romesalis: Total War.