...thinking outside the 'box', how about the buttons (animated on mouseover?) being aligned in a V shape? Roman five, of course, but also Vanguard, etc. Or six far left and right, with the area between having background-blended images fading in and out? Or the same area giving a kind of synopsis of what has gone before (which wouldn't be spoilers, I presume, because the chapter select would only show chapters that have already been cleared) Tricky, given the complex branching, but presumably no more tricky than actually controlling said branching in the game itself.
I have to admit that when I first saw this, I mentally stubbed a toe reading left-to-right.