Western Design Center already sells the 6502/65816 core IP. The 816 already provides a 24 bit address space. It's apparently in more things than you'd guess, they are making enough money to stay in business, anyways. No new IP it seems, but continuing to manufacture the old.
I had a BBC Micro B (a very British computer for those that haven't heard of it) and really wish I'd had this. Sideways ROM/RAM banks just didn't cut it.
Back in the day (late 1980's) there was ample motivation as I had no knowledge of or access to the 65816. Programmable logic has changed the landscape since then -- and btw I have whimsically contemplated a modern re-issue of the KK. (I've also contemplated some KK-ish hacks that would yield an improved 65816!) But I agree the scope for practical application has shrunken. In the first place you'd need to be committed to the 65xx family, because alternative, modern processors offer compelling advantages not featured by 65xx.
That said, KK has features not present on the 65816, and in certain applications these could be pivotal. Obvious examples include the NEXT instruction and the new addressing mode. Less obviously, KK preserves the 65C02's bit-manipulation instructions, which can be a boon in I/O-intensive code. The '816 sacrificed these opcodes to make room for alternative, also-worthy objectives.