I would be okay with a story involving Klingons or Romulans, but not bringing back previous Klingon or Romulan characters. Those are two races that would also be relatively simple to do in terms of makeup and costuming (assuming they stick with the TOS styles). The idea inferred in TOS (re: "The Immunity Syndrome" as well as Spock's constant presence) of other aliens in Starfleet is something I'd like to see bolstered by seeing an Andorian or at least another Vulcan as a Starfleet officer even if it were done in similar manner as they handled Erin Gray's Commodore in "Lolani" by being visible on a viewscreen. It could be fun to see a somewhat light-hearted story in similar vein as "Shoreleave" (although I'm not advocating a return to the Shoreleave planet).I admit there are things I'd like to see in STC, but practically none of it involves looking backward or calling out to other productions.
Were there any rejected story ideas that (in basic concept) could be salvaged and fleshed out into something worthwhile?
Matt Jefferies did sketch out an idea for a largish space station that, unfortunately, never got realized and they went with the K7 design instead. But MJ's concept has since been modeled in 3D (by Mgagen I believe) and it would be cool to see it onscreen given the appropriate story need. STC has already done this sort of thing by using a MJ sketch as the basis for the Tellerite ship in "Lolani."
I'd love to see Doug Drexler do justice to the original Klingon D7 battle cruiser rather than the sloppy work seen in TOS-R. In extent while MJ was sketching out ideas for the D7 design maybe there's something in among those sketches that could serve as the basis for a diferent class of Klingon ship.