I think the title can go two ways. And this is just off the top of my head. I'm sure the design team has thought this through already. So beware of rambling ahead.
The title could be very descriptive like all Osprey books. This makes sense for Osprey titles since they are meant to be informative on their subjects. This type of title is more palatable to the wargaming crowd but I'm not sure if the unknowing public will find "Rules for Ancient Wargames" any more helpful a title than "Art of War" in knowing what the book is about. Do they even know what a wargame is? What "ancients" refers to?
Also this kind of title is used for dry material like textbooks and technical manuals.
The second option is a more evocative title. Like "Warhammer" or "Flames of War" it inspires a sense of the excitement of what the game is trying to create.
I think some of the suggestions so far would be fine if the market was simply the existing ancients and wargaming crowd. Ancients is popular and the layout and such planned will go a long way to getting these rules played a lot in the wargaming community. But THE title has to grab out and make statement that this is THE game to play to grab non-historicals players. Waht about non-wargamers in the bookstore assuming Osprey will get it there? are they a market that is attainable? Will the spine on the shelf at Chapters lure them in? Who will be looking at the spines on the Osprey rack at chapters? What will grab them and make them decide that playing a game based on their interest in history is a good idea.
Part of it is what makes up the rest of the package. Full colour illustrations and beautiful diagrams will go along way. Easy to read examples of gameplay as well. Don't forget other marketing. Will there be any or is the spine on the shelf all the consumer will know? In a print ad "flames of War" is much sexier then " Rules for Wargaming", but on the spine "Flames of War" is not necessarily going to draw interest necessarily.
I started wargaming by accident. A friend's dad had some 54mm pre-painted Napolenics on his bookshelf. I asked about them and he told me you could get them unpainted too. I thought, what fun to paint those since I'm interested in history. Only months later, and a few bad paint jobs, did I learn that people played with them and I was very keen to see it in action. Anyway , I digress, but my point is that playing with toys based on history is the draw here. The title should reflect that, imho.
It could do that with a byline that is emphasized on the spine and cover, "Rules for Battle in Miniature" with the main title something sexier. two birds, one stone.
Anyway, I'm losing my train of thought and it's late but I'd be curious to hear some of the expectations for the title from the designers.