Hummm, let's see.
It'd be a convertible.
With really high top slots and really heigh RF weight limit. Maybe designed like the Swedish seats so at some point you could lift it up from the vehicle's seat to get more leg room.
There'd be a removable top part, like the True Fit, so that while your child is still short you don't need to have a monster taking up front to back space. But unlike the TrueFit you'd be able to use it without the extra part longer, regardless of weight, until your child's head was within 1" of the end of the shell, then you'd add the extra part.
RF tether.
If I could make that work with infinite harness slots I would, but if not regular slots would be ok, as long as the lowest one was low enough to fit newborns well.
Push button adjuster!
You'd be able to remove the cover without unistalling the seat. And definitely be able to remove and replace the harness.
Lock offs. Definitely lock offs. And aligator Britax like latch connectors. And Britax like chest clip.
TSIP, but unlike the BLVD, the wings would be farther from the child's head, so it'd just look like an MA except the part around the head would be deeper so the head was contained. So no headrest piece to push the head forward.
Well, while I'm at it I suppose folding like the Radian would be cool!
And of course, like any self respecting seat, there'd be EPS foam all over and lots of padding. Lots of cover options, including cute boy covers!
Who's buying one?