Honestly, imo all KEF really needs to do is just glue an LS50 Wireless II on top of a KC62 and it would have a near-perfect active bookshelf.
Even as separate cabinets, if you literally just sit an LS50 Wireless 2 on top of a KC62, the dimensions are still just 22x10x12.
That's just a little bigger than a Genelec 8351B(18x11x11), and presumably KEF might be able to shave a few inches if, you know, it actually tried to keep everything in the same cabinet.
By comparison, a pair of the 8351B costs ~$8,000, while a pair of LS50WII + dual KC62s (again just as separate devices) would be $5,500. As tuned out of the box, at least, the KEF combo would have far more bass (and lets face it, most people are not doing fancy DSP tricks to extend the bass extension of their speakers).
Yeah I wouldn't recommend the top and bottom slot vented woofers, they really limit the speakers placement abilities when you have deep bass coming out from the bottom of the speaker, it does seem to cause some unwanted reflections that makes the bass really undefined and boomy with the Genelecs unless you have completely unobstructed placement.
That's an interesting concept for sure, I guess the question is in that case is would KEF do something like that? Aren't most 3 ways build with higher XOs, like around 300-400hz? With a subwoofer driver like that I would imagine it would be best to cross over lower, but at that point people might just wonder why not get a sub.