I've been thinking about this a bit… I know the desire would be to create a modern desktop metaphor GUI, but … do we need or even want that? If an alternative to Matt's 32k giving us a TiPi-compatible SAMS were to be created, or a P-box SAMS card with TiPi, you'd have access to a modern mouse. All for that! F18A Mk 2 should be able to give us the option of more colors, more sprites, more graphics in general.
What I wonder is … what if the text screen and the graphics screen were essentially the same screen? That's not far from how the stock VDP operates anyway, but let's maybe improve upon that. What if a program can take over the screen or part of it to render graphics on top of the text? Again, almost the VDPs native mode, except there's no coordination or standardization.
I think about Contiki, a GUI program on the C64, but plain text for faster speed on the Apple II. But its TUI tries to implement ASCII icons. What if it could just draw graphics there? What if it could use colors to use the text screen more like a GUI screen, and instead of being a character cell MouseText "arrow" or inverted block, the cursor were a mouse cursor that moved like you'd expect?
From where I sit, the desktop metaphor is going away. It's not really used on modern versions of Windows and is increasingly less used on macOS. The desktop is becoming a scratch space for a current project and the usual menus and the like are being replaced by program finders or selectors—pop up screens full of programs to choose from or search through. A setup like this might lend itself nicely to a Norton Commander style file manager, or maybe you could run a modified beginners or extended BASIC that ran under it. Multitasking maybe with a model like screen/tmux, either full screen or in subwindows, or maybe both? And because programs would be cooperating, you could mix GUIs and text. This IMO capitalizes on the fact that the TI, like most 1980s vintage computers, is pretty good at text, but still give you the power to use graphics wherever you need them.
I know I've often wished I could just switch tmux on my Mac or Linux machines to a tab with a GUI web browser in it. I don't often tile things because I like being able to see what I'm doing with large fonts, but a split screen does have its advantages.
Throwing a pipe dream out there.