A device by the name of Mokibo has since been launched on Kickstarter in which it bills itself as an iPad Pro keyboard accessory that features the trackpad built into the keyboard itself. The trackpad is hidden under the right hand side of the keyboard underneath the keys, which means that you can glide your fingers over the keys to access the trackpad, or tap on them if you want to type normally.
According to the creators, there is a touch sensor under the right half of the keyboard where it can apparently detect when the keyboard is being used as a keyboard, and when it is being used as a mouse, so in theory, it should not interfere with your regular typing when the trackpad function is not being used.
https://www.kickstarter.com/projects/mokibo/mokibo-folio-for-ipad-pro-2-in-1-touchpad-fusion-keyboard
The Mokibo is priced at $79 for early birds, but it is expected to retail for $170 when it is released. While it’s not exactly cheap, it is almost half the price of what Apple is charging for its Magic Keyboard accessory, so it could be worth checking out.