Honestly I think this post should be lots of separate threads, isn't that what the sub-forum is for? But, since it's here, might as well participate.
This isn't so much a suggestion, as something to think about and maybe a question. The reason I enjoyed Civilization was that it allows you to decide HOW you want to win the game. Civ V seemed to undermine that, from my perspective having played nothing more than the demo, by making combat a huge mechanical focus, even more so than in earlier games.
But playing as a Barbarian Tribe during the fall of Rome is different. And I'd like to know, specifically, what your design philosophy behind this game is? What decisions do you want us to be making? Is combat with the Romans 'inevitable'? Will there be more than one way to 'win the game', or in general what will the win states be?
A lot of this I'm sure you're going to cover, but it's hard to give specific advice when we know so little about the game! I suppose my point is just know that you WILL be compared to other games, especially Civ, so the game needs to have its own mechanical ... 'Identity' I suppose, and that identity should be pretty clear - and it's hard to give specific advice until we know that identity.
I mean, I have plenty of ideas, but this is, after all, your game, not mine. I didn't pay enough money to say 'make this more like Civ', assuming I want that (I'm not sure honestly), and I wouldn't try to do your job for you even if I HAD gone higher up.