Chen, what is your opinion about a trackpoint either on the keyboard, or on the back side (if I remember correctly, the PSP or PS Vita had a trackpad on the back)? It could be something like the mechanical trackpoint of the GPD Pocket, or typical IBM laptops, or the optical trackpoint of the GPD Pocket 2. I heard mechanical trackpoints are better but I never tried optical.

GPD 1, mechanical


IBM, mechanical


GPD 2, optical


HP Mini Wireless, optical + clickable


Left and right click buttons do not have to be as big as on the GPD 1, nor do they even need to be located next to the trackpoint, and the trackpoint itself could be placed elsewhere, as on the GPD 2 (third picture) to keep the keyboard layout unchanged. Actually having the trackpoint on the top right, next to resting position of the right thumb would even be better for a device designed for thumb typing like yours. Ways to avoid adding new buttons for left and right clicks could also be imagined, like using Vol+ and Vol- as mouse clicks when the keyboard is open (as long as the left index can reach Vol+ and Vol- easily), or key combinations to be done with the left thumb like Super+Z and Alt+X (those are based on your mod's layout, since both combinations could be done with just the left thumb and no other finger, but it would have to be adapted to the new keyboard of course).

If the device turns out to be a multiboot device capable of booting a full fledged Linux OS, like the Gemini is with Debian, then being able to use the keyboard and the mouse without plugging anything would be a key killer feature. One could use Debian in the subway, in the train, in bed, whereas needing an external mouse is impossible in those situations. A touch screen is of course mandatory for a phone, but it simply does not replace a mouse/trackpoint in terms of productivity, unless it comes with a stylus like the N900 (and a capacitive stylus will not offer the same accuracy). Many laptops now have touch screens, I had one myself on a 12" laptop, and I quickly realized I would never use it to replace a mouse.

The Gemini doesn't offer a trackpoint and that is one of its weaknesses. The GPD does, but first released pictures of the GPD 2 showed no trackpoint, and they caused a great concern among users on social networks, to the point that the company altered the official pictures to fit in the optical trackpoint and show that they will address this concern. This shows how much people interested in HWKB handheld devices care about having built-in cursor control, without requiring an external peripheral.

For LineageOS or Sailfish, the benefits would be limited, but for other OSes, this could be a game changer, with little or even no alteration to the keyboard layout and thickness.

[Edit] "Trackpoint" seems to be IBM's wording, "pointing stick" is the generic keyword.
https://deskthority.net/wiki/TrackPoint

[Edit 2] I would gladly trade any of these pointing sticks for a N900 stylus, but I doubt we can reach the same accuracy with a capacitive stylus, and the stylus holder of the N900 is not to be negliged: it makes the stylus always very easy to pull out or store in the blink of an eye, with just the right index.

[Edit 3] Or the brave-hearted can glue an Ergo Mini Touchpad on the back of their Livermorium I guess :