About how long the old artwork will be available, I'm not so sure about that. It seems, some of the main expansions are already out of print. I think, they will be more likely be reprinted in the new style, but I could be wrong. At least the next small expansion seems to be "old style". Nevertheless, when I bought Carcassonne Big Box 4 last autumn, I was first planning to buy no other expansion, till the family and me getting tired of the stock presented in this BB, then adding one at a time. Because of the change of style, I reconsidered and now I have nearly everything I want and hardly anything of it played yet. (Or only played on JCZ.)
About your proposal: I think, you could elaborate it as a variant. It will always be a kind of house rule or variant, since the official rule is, that all tiles are equal to use, not regarding the type of artwork. I'm not sure, how the mix of roads and fields will look like. But It would be interesting testing nevertheless, which and how many base game tiles, which expansions in which style would be best to mix, to make the game balanced and fun to play.