My $.02;
Personally, I like the current art style of AI War. It's unique, and somehow, fitting. And to be honest, the majority of the time I'm zoomed out enough to where I don't even see the art. I feel it wouldn't be worth the money and effort. I didn't buy this game for the graphics, I bought it for the gameplay. I'd much rather see the latter improved than the former.
That said, I feel there are some ship designs that could be revisted when changing from Mk1-2-3-4-5. Mainly, keeping them in line with the base ship but adding on... more, as opposed to drastically changing the model. I can't really think of any ships that are like this off the top of my head though, hah.
But that's a minor issue, and a personal one at that.
Also, I'm loving the planet/space backgrounds in this game. I love how non-standard a lot of the planets are. If not done so already though, I think it would be cool to see random scaling for the planet elements in the background though. Ranging from very small to very large in terms of how much screen space they take up. Why? Extra, easy variation sounds good (and I think some of the planets are... slightly large. Again, a personal complaint).
Lastly, as much as I love AI War, I couldn't support AI War 2 at this point, if it was coming primarily due to a desire to change the graphics. I do not like the idea of splitting the player base more than we have to, and I have no problems with the game the way it is. I would be, however, willing to support a larger than normal expansion (or any expansion for that matter), even if that meant the price would be a bit larger (as long as the content:price ratio is reasonable).
[Side thought: If the current engine/design of the game does not allow you to follow your vision for the game and what you'd like to add, then maybe it might be time to work on AI War 2. I would just hope that there'd be some form of price reduction (wouldn't have to be major) for owning the base game as well as a small reduction for each expansion owned. I unfortunately don't have as much money as I would like to spend on games and such. AI War's prices have been fair I feel, so I wouldn't be too worried. Also, I would try to stay away from 3d models if possible, I feel there would be little benefit unless the game went full 3d, plus, we don't need more strain on peoples pc's then there already can be. :p]
Long and short of it is, I like the graphics, and I think they fit the universe. I would be opposed to spending money on a graphic-overhaul, but I can support purchasing future expansions.