I don't dispute that the rollbar has its problems; several posters on different boards have brought it up. It's my only significant addition to the basic design, and I don't think aridas
is in love with it either. I just haven't come up with a better solution so far to what you noted as the "incompleteness" of the core - and I don't think it's a matter of incompleteness at all, just that the ship needs something to build it up vertically so that it isn't diminished from so many angles over than above/below.
Vertical towers or fins are the only other idea I can think of. There was a variant by another poster using a broken ring which I kind of liked, except that to maintain the visual balance of the design the ring pretty much had to rise from the centerline of the engines and I couldn't make design sense of that to my satisfaction.