I think TOS centric Short Treks like the Uhura one are likely, if Short Treks ever come about in the future, because they would be a fanciful bridge between Strange New Worlds and Picard.
Nichols is not in a good position, but Takei or Koenig could fill a similar spot, especially if young Sulu or Chekov are ever planned for SNW.
But, yeah, we've had 10 Short Treks, and they all fall in the same basic categories:
- Discovery stand-ins: Runaway
- Experimental Ideas: Calypso, Ephraim & Dot
- Prequel tie-in: The Brightest Star, The Girl Who Made the Stars, Children of Mars
- Fun, independent stories: The Escape Artist, The Trouble with Edward
- SNW Pilots: Q&A, Ask Not
The main thing they all have in common is their existence as a tie-in to a current show. A cameo from a TOS figure or Berman era character would likely have to tie in to SNW or STPicard in some way.
I think, at least in terms of STPicard, a fun cameo is more likely for Short Treks than the show itself.