We've been hard pressed to find major updates so far, but maybe Apple snuck something fun onto the logic board:

Apple A10 Fusion APL1W24 SoC (also found in the iPhone 7) with 2 GB Micron MT53B256M64D2TP-062 L XT:C LPDDR4 SDRAM layered beneath

Apple 343500203-A0, likely a PMIC, possibly a revision of the iPad 5's 343S001441-A0

2x Broadcom BCM15900B0 touch screen controller found in the 10.5" and 12.9" iPad Pro

Our guess is the new Pencil support comes in part thanks to this "Pro"-grade chip.

NXP Semiconductor display port multiplexer (likely)

Bosch Sensortec pressure sensor

Invensense accelerometer