I would be interested in seeing some first contact stories for some of the major races, like the Trill, the Betazoids, Bolians, Catians, ect.
Agreed. There's great potential for interesting exploration/diplomacy stories centred on worlds that will go on to join the Federation. The fact that these would be cultures with a confirmed future role to play, rather than random planets-of-the-week, would add greater significance to the proceedings, and it would be a chance to really flesh out some of these races.
How much do we really know about Saurians, or Grazerites, or Arkenites? Only scattered tidbits. And a political piece detailing the untold story behind Betazed or Efros' entry into the UFP would be fascinating.
After Rise of the Federation
explores the earliest years, I hope we'll see novels - or even a series - about the blossoming of the Federation and its swift expansion. When we "last" saw the Federation it was a fledgling nation with five member states, when we "next" see it in the mid-23rd century it has about a hundred members, some quite exotic. I'd love to have some of the stories told, especially as it would satisfy both those wishing to see more "pure" Trekkian tales (exploration, discovery, new worlds) and continuity fans (who would get to see the history of established planets developed further). In fact, such stories would be a great way to satisfy fans of continuity without bogging the novel down in the minutiae, making it easily accessible to casual readers also.
Can you tell I'm enthusiastic?
Oh, and someone take us to Denobula, for goodness sake!