When I think what I'd do for a Rome expansion to make it more enjoyable, the first thing that comes to mind is obviously a complete reorganization of the map provinces (smaller ones and sea-focused), even if the map scope doesn't move eastwards.
Then, another important sphere of the game that needs a complete revamp is the military system; for what I've seen the main thing that counts in the military aspect is the number of troops of each nation tag: of course it matters, it wouldn't be realistic if it won't, but logistics, attrition and possibly terrain should have a lot to do with the chances of some country even thinking declaring war on another, even if it's forces are underscored 3:1 or 4:1.
But what I think could make the game more enjoyable is about the character system; why CK is so fun to play and Rome doesn't? The first thing is that is very tedious, and it takes a lot of effort for the player to fully know your realm. In CK you can easily know which is your ruler, your family members and your courtiers. After playing a single afternoon you can almost imagine that you live and know all of their lives, goals and miseries. Why this doesn't happen in Rome?
What I'd do to solve this would be something like this:
I don't know if I'm the only one, but I find it difficult and tedious to access the characters screen in the bar above; when I do that it's difficult to get the big picture of your court; you must scroll down through men, women, children. And of course there are filters, but it's still somewhat tedious to check.
I think the CK system of small portraits down the characters screen would be absolutely useful for this purpose. Plus, I go a step beyond: since there are political parties in republics, tribal factions and heir supporters (by order of loyalty), wouldn't be useful to have various small portraits of the people supporting each faction?
Big countries like Rome could have this bar extended twice so they all fit in, and why not, in the right edge of the characters screen it'd be nice to have a couple of icons to filter men/women-children or employed/unemployed showing up in that mini-characters ledger.
Ah, and a minor thing could be to have dead-spouse/brides visible like in CK. It's a very minor feature, but I like to give some honour to the unfortunately gone brides.
o