Big as a 50-70″ TV. Big enough to put on an easel or mount on a wall. Lets call it the Canvas. (Finally a purpose for those 8K displays that are otherwise pointless except as hi-res computer monitors.)
More than pressure and tilt sensitive. Shape and detail sensitive. So I can take my (clean) well-worn favorite old paintbrush, use it directly on the tablet, and have every detail of its bristles picked up: Individual shape of each bristle, each separate amount of pressure and change in direction.
Fast enough to do all the calculations required in paragraph 2 in real time, along with whatever the painting app does in applying all that to its digital paint, etc.
Give it a separate small physical pallette I can hold in my hand, or set on a stand or table, for mixing colors! That should have paragraph 2 and 3’s capabilities when mixing colors, too.
Make the little pallette the item that triggers popup menu, maybe with a handy physical button. And popup menus could optionally appear on either the pallette or the Canvas. (On the Canvas they’d be translucent so I could see what’s beneath them.)
That would be my idea of a tablet.
OK, Kickstarter engineering whizzes, have at it!