it will have benefits over the 4.x - we can always spin up a version with the iMX chipset (we have a metro board with the little sister chip, iMX RT1011 already in stock) - tbh if we did something with the iMX RT106x we'd probably start with a Metro (Arduino-shield compatible) or Feather board since that's a super-popular pinout.
either way, more hardware is better and we don't want to just give people the same-old-same-old... as we mentioned there's lots of things that we can add to make the board useful to people: SWD, USB C, Lipoly batt, onboard storage, neopixel LED, etc). what peripheral/library are you specifically concerned about?
Mostly I'm just leery of software defined peripherals being at the mercy of whatever community springs up around them, nothing specific. In terms of a Metro then yeah, something to slot in where the Due was absolutely with high speed USB, 10/100 ethernet, CAN FD, and all that jazz that wouldn't work on a $10 board. A SAMV70 successor to the Due?
NXP just seems antithetical to an open platform. Then again Arduino went with Renesas, and they're… not great.
Otherwise it's the openness that would pique my interest. SWD headers, yes 100%. But also the documentation. No half-assed SVDs, buggy closed source flash algorithms (Microchip), wholly undocumented peripherals (looking at you Renesas), stuff like that.
If you replace the Teensy 4.x it would have to be something very close to the same pinout, foot print, cost and features otherwise it would just be a new product. Ideally you would find a way to source the Teensy directly bypassing Sparkfun.