Like others here, I was excited to see this thread and the implications it seems to bring (GR happening soon?!?), but I didn't want to just jump in and give my two cents. I figured I need to give it some time for the question to sink in and really come up with the best idea.
Like others, I would really like to keep my hands on the keyboard as much as possible. So, some sort of marriage of the textblade and a pointing device seems like it would be best.
My first thought (like @Jesse_S mentioned) was to put a track pad in the space above the keyblades, but I had that thought when I wasn't near my textblade. After sitting down at it and trying to see what it would feel like to try and touch a trackpad above the keyblades, I found it to be uncomfortable. It's too much of a stretch (and one of the reasons why they engineered the textblade to pull down the numbers onto the top keys).
I then thought, maybe they could put one of those nub things like they used to put on the old IBM ThinkPads (oh, that's called a Trackpoint) right in the center between the keyblades. However, I quickly dismissed that idea. I can't imagine those being very precise. I never had a ThinkPad and only maybe used one once trying to help someone with their computer.
So then I tried to think about what I do now with my beloved Apple Trackpad (not one of those new ones, but one that actually clicks down when you push it). I move the cursor around with my right index finger, and if I need to click on something I either tap or press down to click depending on my mood (though like I said, I like the physical click). If I need to click-drag something, like a window or a file, I use my right thumb to click down and my right index to move the object. I do the same right-thumb-click, right-index-move operation in apps like Photoshop. It serves me well.
So, if that's how I use a trackpad, how could I translate that to the textblade? Well, I already use my right thumb for hitting the spaceblade, so perhaps that should be the mouse click. And my right hand (and thus my right index finger) is already resting on the right keyblade, so maybe that could be my trackpad surface. I would just need some way to make the textblade know I want to move the mouse. Well, we already have precedent there too with all they chords for edit/select/ctrl/alt/cmd/etc. so why not just have another chord for telling the textblade that I want to control the mouse. Maybe like holding down WER (three fingers) or QWER (four fingers).
So, my proposal would be to just use the Textblade .. no new hardware (or at least nothing visible, there may need to be some changes internally). So, you would activate mouse mode with a chord, use the spaceblade for the mouse button (could even support primary, secondary, tertiary clicks depending on where you pressed down on the spaceblade), and use the keyblades as the trackpad surface. One finger for moving around, two fingers for scrolling, etc.
Now, I know there isn't much surface area on a keyblade, especially compared to the massive trackpads they are putting in macbook pros these days, but it seems like it would be fine, you may just have to pick your index finger up a few more times to move your cursor all the way to the top or bottom of the screen. I suppose you could have an alternative mode that just used the YUHJNM key for movement and have the cursor move faster the closer your finger gets to the edge (sort of a mix between the Trackpoint mouse and an Analog Joystick). That might work, but would take some getting used to.
Anyway, I know that this isn't a completely new idea, that it's been proposed before, but it seems like all the elements are there. It would at least be something to experiment with.