Okay,
your choices I think all should be included, to make the best vac.
I don't think you have to have massive power, a good BB should cause a lot of the stuff to be sucked away, I would like to see a little longer brush to help dig and flick the deeper stuff, even if didn't make it into the vac, it would be much easier to take it away when we did the carpet cleaning.
A BB that didn't get bound up with hair in the bearings would be nice too.
So if what I said was done I would say good filtration to keep the fine stuff contained and not in the air.
I have to say I have really liked the sharks I have, the BB is a little weak but the ability to go from vac to edge is really really handy, the debris I am getting is very good but I have noticed it seems to be the lighter debris, like fuzz, I honestly can't say I have emptied it and said wow look at all that sand or heavy debris. Like I get with the Red n Chrome