I fear that engaging this lot will end up in the same mess that plagued Jolla. The too many cooks syndrome. But what the heck, here's my tuppence.

For starters, I think a tablet is much closer to the desktop than to a phone. I think the major mistake all tablet OS producers make is to use the same UI on the tablet as they do on the phone. So perhaps the best starting point would be to look around the existing desktop environments and seeing if any of them could be suitable on a keyboardless touchscreen device with minimal changes.

The second thing I would suggest naturally comes from the first anyway but I will spell it out just in case. Focus on function, not the form. No facy animations that slow down the work flow. No unnecessary transparencies. Really count the number of taps (swipes, key presses...) it takes to do each task.

Case study: the N900 keyboard. I know, I said keyboardless. This is just for illustration. I do not know about other language variants but the English keyboard has the dot in the main layout. No shifts, no symbols. So far so good. BUT - consider a text entry box that accepts only numbers. The numbers on the N900 keyboard are on a symbol shift, which is helpfully pre-selected for you when such an entry box receives a focus. The trouble is, the dot (in this case, the decimal point) now requires pressing the sym shift key. Annoying as hell. Good old Palm did that just right: the dot was a dot without a shift in both the letter and the number mode. Why? Because they cared about workflow down to the minute details like that.