Indeed, too early to talk seriously about this.
But I'll briefly repeat what I've said before, that we'll probably wait for a "larger" chip from NXP to use on "larger" board.
While I can't talk about any future NXP products (which would be covered by NDA until NXP publishes info on their website), you shouldn't need to use much imagination to guess they'll likely add more iMXRT chips over time. That's the path Kinetis followed, and pretty much standard practice for every product line of microcontrollers.
If you were to imagine what such chips might look like, you might take a cue from the fact that iMXRT is pretty much Cortex M7 plus a mix of peripherals from iMX6 and newer Kinetis parts. They have lots of other chips on their website with pretty amazing peripherals.
In a few months, after Teensy 4.0 has stable stock levels at all distributors and we've released Teensyduino 1.49 or 1.50... then will be the time to talk of form factors and I/O. Until then, please feel free to discuss here on the forum. But don't expect me to be involved in any serious way until we've got Teensy 4.0 inventory levels stable and at least a couple software releases made.
At this very moment, I'm working on speeding up the Arduino Serial Monitor. That's on my high priority list for 1.48, so Serial.println("Hello World") without any delays won't crash Arduino on Windows & Mac (and make it run unusably slow on Linux). That is where I'm focusing my attention right now. That is the reason you're not seeing me on the forum much this week, and why I'm not willing to talk seriously about next Teensy feedback... at least not right now.